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Free Long Distance Calling Using SIP (with Linksys PAP2 or any softphone)

Short and simple steps for the technically challenged

For this setup, you will have to first buy a phone adapter. A very good and cheap SIP phone adapter is the Linksys PAP2. This will cost around Rs.2500 in CTC, Secunderabad. If you want to use single phone for internet calling and normal telephone line, then you can consider buying Linksys SPA3102. (Note: Newer versions of these ATAs modelled SPA112 and SPA122 have been launched). After you buy the phone adapter, follow the below instructions to configure cheap calling to international destinations:

  1. Go to and create an account
  2. Get the IP address of your PAP2 device by following Step 1- Step 5 in
  3. Configure Voipdiscount in your PAP2 device by following the instructions at See section on Linksys SPA3102. Also see section on "If you have audio problems:".
  4. For US number, visit Create a new phone number and assign the below SIP address.
    SIP Username: Your VoIP discount username.
    SIP Server:
    For UK number, visit and register your SIP address : (substitute username with VoIP discount username).
    For more such DID Providers, visit
  5. (Slightly Advanced) For direct IP calling, follow the instructions in

For understanding more about SIP, read on.


This is the second article I am writing on Long Distance calling. And in a way it obsoletes my previous article on Net Telephony. This article mainly focuses on using SIP for making long-distance calls. SIP (Session Initiation Protocol) is a protocol which is used for voice communication for internet (Read more). For making these calls, the bare minimum is a net connection. The setup is best explained with the below diagram.

SIP Setup

For furthur clarification, an analog phone is just an ordinary corded/cordless phone using which you place your regular landline calls. Now that you have an idea of the setup, let us get into some real action.


Choosing a SIP provider

Just like when you chat on yahoo messenger, you have a yahoo client installed on your PC and Yahoo Server providing the messenger connectivity, for SIP you need to have a SIP provider (and SIP client). I had considered many SIP providers. I have summarized some of them below.

  • Callcentric - The biggest reason for me to try out Callcentric are its really good reviews at Callcentric Voip Reveiws. And true to the reviews, the quality of Callcentric as of today seems to be really good. Also their rates and packages they offer such as Pay Per Call are useful to the end-users. And definitely not to forget the excellent documentation they have for setting up your SIP device which you shall get to hear of soon.
  • VoipDiscount - Voipdiscount is perhaps the cheapest VOIP provider out there on the internet. It is owned by Betamax which also runs many other such similar VOIP services. You can find out more about Betamax and its products at and For configruing Voipdiscount with your SIP device, click here (you shall read more about what a SIP device is later).
  • sipgate and Voipfone - Though sipgate and Voipfone do not offer as cheap outgoing rates as Voipdiscount, they both provide a free UK number. While sipgate offers a number starting with 0845 or 0870, Voipfone offers a free number starting with 056. This makes it extremely useful if your primary need is to have your relatives in UK call your phone adapter.

This was just an overview of some of the free SIP providers. They are many out there which you may feel to try out if you have the time and patience :). I intend to keep this updated as and when I come across any good SIP providers. So for now, we shall go ahead with Callcentric.

Creating a SIP account

The next obvious step would be to create a new SIP account. Kindly visit Callcentric or the website of the SIP provider you have chosen and create a new account.

Please note that if you make only (or mostly) outgoing calls, then Voipdiscount is what I would suggest you look at.

Configuring your SIP client

Just as for yahoo messenger after creating an account you have to download and install the client, similar is the case with SIP. You have just created a SIP account and now you need to download, install and conifugre the client.

You can use the free SIP phone X-Lite as a SIP client. There are many other SIP softphones available but I choose this because it is a very popular SIP phone. Also you can find step-by-step detailed instructions for configuring Callcentric on X-Lite here.

Communicating with your friends

Now ask your friend or whomever you want to voice chat with to follow the same procedure by creating a SIP account with some provider, download, installing and configuring the client. Once he does that, either of you can call each other using the SIP number associated with the respective accounts.

Advantages of SIP

You might have observed that what we have achieved so far (i.e. PC-to-PC voice chat) can be achieved using Yahoo Messenger or Skype or some other voice chat software. But what makes SIP so special is that it is based on open standards which enables use of the following features.

Using SIP phone adapter (ex. Linksys PAP2)

This is perhaps the best thing I like about using the SIP protocol. You can replace your computer + headset with a SIP phone adapter + any analog telephone. The phone adapter is especially a boon for those who are not familiar with how to use the computer.

A very good and cheap sip phone adapter is the Linksys PAP2. I recently purchased this piece from Bhumika Computers (in Hyderabad, India) and it costed Rs.2800. As per my knowledge, the latest version as of September 2007 is PAP2T and it comes in a properly packed blue box. Also make sure that this is an unlocked version which you are purchasing. The instructions for configuring callcentric account can be found here.

Also note that there are some other slightly costlier phone adapters / VoIP Gateways which provide additional functionality compared to Linksys PAP2. For instance, the Linksys SPA3102 integrates with your PSTN network thereby enabling you to use only one analog phone for PSTN and VoIP. This device also has the additional provision of configuring multiple outgoing providers and switching between them based on your destination number. If you call up various countries, this would be particularly useful.

This kind of feature of choosing outgoing provider based on destination is also provided by services such as Voxalot and SIP Sorcery. But there might be latency issues when compared to configuring the outgoing providers directly in your phone adapter.

If you have understood the working of the phone adapter, then you must have realized by now that if you have only one network connection, it is not feasible to use your computer and the phone adapter at the same time. To overcome this you will have to buy a device which provides routing functionality and phone adapter functionality OR buy phone adapter and some other router (netgear, linksys, softrouter, etc.) separately. The latter option is what I recommend because it will prove more economical to buy a separate router taking into consideration the features provided by a separate router (such as wifi, multi-port, etc.).

Calling via traditional PSTN networks

It is possible to reach your SIP phone via traditional PSTN numbers. This is made available through the SIP Broker service. Apart from this, SIP Broker service can be used for making calls between SIP networks. Visit their site for more information.

I shall explain how this feature has proved a boon to my aunt who stays in India (she has a callcentric configured on Linksys PAP2 and let us assume her account id is 1722221224) and how her daughter who stays in England calls her via her PSTN phone.

i. The daughther first selects a number from the list in SIP Broker PSTN Access Numbers. For her, all calls to London are free. Hence she selects a London PSTN access number.

ii. After dialing the number, she hears a welcome message asking to enter the SIP number followed by the hash sign. Now she punches in the SIP Provider code which is *462 for Callcentric followed by my aunt's SIP number which is 1722221224, finally ending it with a hash (#) sign i.e. after she hears the welcome message, she punches in "*42617772219612#" (without the quotes).

iii. That's it. Once she does that the SIP phone at my aunt's place rings and they can talk to each other.

Calling from other SIP networks

It is possible to call callcentric users from other SIP networks. For example, people using Vonage / Packet8 should be able to call you directly using this method. For more details refer Callcentric FAQ.

Incoming Phone Numbers

Using the service offered by IPKall, it is possible to have a free local US phone number in the washington state. Please note that for callcentric users, your SIP number will be of the form "17770000001" and your SIP proxy is "".

Callcentric offers paid local phone numbers in other countries as well. For the details, visit Callcentric Phone Numbers.

SIP Broker Alias

For certain SIP providers, you are provided with a SIP account such as but no SIP number. These would pose a limitation if someone would like to call this user using their phone as phones support only number dialing. To overcome this limitation, SIP Broker provides an excellent service which allows creating a free SIP number alias for your non-number based SIP URI. For furthur details, visit SIP Broker Alias

Another way of calling non-numeric SIP address (such as, is by switching to advanced view in Linksys PAP2 web interface, clicking on User 1 (or User 2 depending upon your line) and setting one of the Speed Dial numbers to the SIP address. Then you can call this SIP address by dialing the speed dial number (2-9) followed by the "#" key from your phone. But this does not provide a SIP number as in the previous case.

Inbound Calls Directly To Your Phone Adapter and Direct IP Calling

If you have a public IP / can set up one of the ports in your router to be forwarded to you phone adapter, then you can set up inbound calls directly to your phone adapter. This implies anybody can call your phone adapter using the public IP with which it connects to the Internet. The biggest advantage of this is that people who call you can directly call your phone adapter bypassing the intermediate SIP provider. A combination of Voipdiscount (for outgoing calls to traditional phones) and setting up your phone adapter to accept ip incoming calls proves to be really economical. To enable dialing the phone adapter IP using a SIP number, you may either use a SIP Broker Alias (details above) or Enum (details below). You can also use IPKall (details above) to map a U.S. phone number directly to your phone adapter IP. For more information on Inbound Calls to Phone Adapter, visit this article Inbound Calls Directly To Your Linksys Or Sipura on SIP Broker Wiki.


From - " is a public enum directory of telephone numbers that can be reached over the Internet by anyone anywhere. The system works by publishing a DNS zone, '', that can be used by various Internet applications. The idea is to be able to map your phone number to an Email address, website, VoIP addresses, etc.".

For ex. this is how I can use it. Let us assume my home phone number is "+91 40 123456", then using I can map my home phone number to the SIP address "". Now someone who knows my home phone number can contact me using the SIP Broker Service with the code for enum (*013) and my phone number (9140123456).

Using the Phone Adapter as Intercom

Alright, this you might not find documented anyway. But this is one small little trick I found out. As mentioned above, it is possible to call the phone adapter using the IP assigned to it and port number. As Linksys PAP2 phone adapter generally comes with two phone lines, it is possible to call up one phone line using the other. I'll explain this using a small example. Assume your set up is as below.

Phone Adapter IP:
Line 1 User: user1
Line 2 User: user2
Line 1 Port: 5060
Line 2 Port: 5061

Go to " -> switch to advanced view -> User 1" and set "Speed Dial 2" to "user2@". Similarly go to " -> switch to advanced view -> User 2" and set "Speed Dial 2" to user1@

Now either of the users can call the other person by using the Speed Dial feature of Linksys PAP2 adapter i.e. by dialing the Speed Dial number to which the address is assigned to (in this case "2") followed by the "#" key.

Other Useful Links

Summing it all up ...

VOIP i.e. the ability to transfer voice over the internet has become a big market today. Merged with standard protocols such as SIP, we are getting the convenience of the traditional phone at a very low price. Looking at the current trend, it might not be long before the traditional phone lines are replaced with IP phones.


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