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My client needs to use an android cellphone as an asterisk channel/Gateway. His aim is to make phone calls using his Asterisk PBX through a cellphone running android.
You are free to choose how to implement this feature. All that is required is that in the end the Android phone will be acting as a SIP or IAX to GSM channel/trunk/FXO and other SIP devices can connect and make phone calls using this.
You choose the best way to do it:
Add the feature to SIPDroid: http://code.google.com/p/sipdroid/issues/detail?id=864
Add the feature using the BatPhone Asterisk Build: https://github.com/carlosdesouzamoura/antroid-trunk-sip-to-gsm-gateway-batphone
Or do it your own way.
Basically, this feature would allow Android to act as a SIP "Trunk". Other SIP 'devices' would be able to initiate calls over the GSM connection on the phone, and receive incoming calls from the GSM network.
I can see being required is a modification to the code to route the VoIP audio to GSM audio, event handling for incomming GSM calls, and a small extension to the SIP stack. I would have a look at this myself, but I am having trouble getting my head around the source at the moment (I know very little about Java or Android programming, and I have always hated trying to understand someone else's code anyway) although I am willing to keep trying. If someone can point me in the right direction I will make an attempt to hack the support in as a starting point.