How to make calls through an android smartphone/3G modem from a local network in a small office?

Question

SIP

Given: Several linux / win computers in the local network and an android smartphone / 3G modem.
Do: A simple and reliable solution to call via android smartphone/3G modem. Maybe there is something like a web client for this?
In which direction to dig?

Answer 1

Buy a gateway 1-4 GSM slots, for example GoIP4. Install softphones (microSIP, linphone) on the PC. Connect everything to the cloud PBX and make calls.

Answer 2

dig here