Android :- VOIP & SIP

Mostly we  use  PSTN (public switched telephone network) for making and receiving the voice calls and pay the call charges as per call plan.  But with the use of VOIP for mobile or mVOIP we can set up our own small  mVOIP (mobile voice over IP) network  and communicate with our group members  (pbxes.org) or can call anyone having VOIP enabled phone .  VOIP over mobile phone  is achieved  using different kind of protocols , but most popular SIP (session initiation protocol) is the most widely used standard. VOIP uses the SIP standard for signalling and controlling the voice or video over IP (Internet Protocol). 

SIP is peer-to-peer protocol so it requires very basic network over internet. VOIP converts the analog voice signals to digital , creates the data packets and transmits the packets over internet. SIP plays the role of signalling and controlling protocol for these data packets.

Like me if you have a droid and access to Wi-Fi ,  you can install a SIP client and make  voice / video  call using the Wi-Fi.   SIP client installed on phone makes the phone act as  SIP phone. SIP protocol needs the SIP clients , which are SIP network endpoints to communicate. As SIP is peer-2-peer protocol so we do not need any other network element in- between, but SIP service providers will put a SIP server in-between which will act as proxy server connecting different clients with each other.

To get a SIP number you can create a account with any SIP provider  (ipetel,   tpad, worldnet etc) .You will get a SIP number , password, SIP proxy. I used ( https://secure.tpad.com/signup/) to get my SIP number .  For android the best available SIP client is Sipdroid , it is open source SIP application and is generally available via gnu licence.  Go to android market on your phone and download sipdroid . After installing Sipdroid , start the sipdroid . It will ask for phone number and calling party address, it also give option to create new pbx account and link it to your google voice account. You can create a account with pbxes.org or you can use sipdroid as normal SIP client without pbx account associated with it .

To use Sipdroid without pbx account go to settings and select SIP Account .  Provide the SIP number for authorization name field, type in your password for password and enter the SIP proxy for server or proxy field.  All these information was available to us from SIP account creation with SIP provider. Check mark the use WLAN if you are using Wi-Fi .
After configuration, if you connect to Wi-Fi you will get a green dot in your notification pad on phone , confirming the sipdroid online.  Now to connect to any other SIP client just dial the SIP number and get connected.  SIP clients provide us with free VoIP calls over IP , condition phone should be  IP enabled. 

If you have fring account , you can download fring mobile app for android which is free and also can use it as SIP client along with normal fring calls.