Hi,
I thought I would suggest an SDK (Software Development Kit) that I found recently that works well with most PBX. It is called OZEKI VoIP SIP SDK and it is compatible with FreePBX, Asterisk, 3CX, Elastix, PBXpress to name a few. Here is the website: http://voip-sip-sdk.com/p_43-how-to-setup-ozeki-voip-sip-sdk-with-freeswitch-voip.html
I think it is a good opportunity for a developer to create a customized VoIP application, and I would suggest it to any professional, who is interested in VoIP technology.

Cheers,
Michael