Dear
Already done similar project. Here is my Sipdroid customization project:
https://www.freelancer.com/projects/asterisk-pbx/Mobile-App-Java-Voip/reviews
I have rich experience on SIP, FreePbx, Asterisk, XMPP calling.
I have a broad knowledge about networking and wireless. this includes routers, firewalls, access points(cisco, mikrotik, ZTE... I am confident that i can deliver any work satisfaction guaranteed through Quality and Effectiveness.
I'd love to hear from you.
Regards,
WanJing