Sir I'm Mobile application developer and I have team for backend development of web and VoIP both platform.
we all are experience Developer and we have completed many projects with work together.
I have developed 5-6 VoIP base App. so I have very good idea about VoIP base app. also I have very good VoIP developer for server side development.
I have user linphone and XMPP for client side and I have own customised sip product and our VoIP developer is used Asterisk, openSIP, freepbx, Free-Switch and so on for server side.
we are not use pjsip because linphone is better than pjsip . we will provide you best voice& video quality.
also we have developed app like WhatsApp and Skype using linphone and XMPP.
please contact me I will give you 100% result.
thanks