Sip to GSM

I need to use an android cellphone as a channel/Gateway to make phone calls using a PBX through a cellphone running android. 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. 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 want to send voip traffic to the application and it will connect this traffic to the gsm network. In details I will put a gsm sim card in a android telephone, I will install the application, I will link the application to my server, I will send voip calls (in sip protocol) to this application witch will connect the call to the GSM network to reach the destination (in telecom its called to terminate a call). It could be 4g, 3g or wifi not necessarly 4g. Its a kinf of soft gateway, normally this work is done by gateways in witch you will insert sim cards, I want to buil a soft-gateway to have this work done by software directly true the smartphone.