Salut!
I'm a linux admin/user since 2001,I have long experience with RaspberryPI, OrangePI's and other mini PC based on linux. Furthermore, ever since 2007 i work on VoIP systems which includes Asterisk/Free-switch, FreePBX,PowerPBX and A2billing.
I would love to work on your task to setup Asterisk/Freepbx & integrate USB Mobile modem on PI.
Shall you have more requirements i would love to brain-storm the details.
Looking forward to work with you, Narok !
Mathew