Build your own Asterisk Server
 

I'm surprised that this forum has no mention of the "Asterisk for Raspberry Pi" project.

Asterisk for Raspberry Pi

This project explains how to build an asterisk server. The hardware is inexpensive and it is easy to connect a sim card in a USB modem, to a Raspberry Pi.

I always take my own pocket-sized, asterisk box with me when I travel.

And this is what you can do with two Raspberry GSM gateways...

http://blog.carrier-connect.com/

I recently picked up a WRTP54G wireless router for cheap and figured I'd end up with an OpenWRT router with a SIP server/client. This router has two VOIP phone ports. Turns out that a few people have talked about it but no one has gotten anything working. I don't have time to invent the wheel.

I have five active Raspberry Pi GSM gateways and they're quite stable. The design of the SBC is in my opinion not very well suited for this purpose. The current version relies on micro USB power and suffers from constant power fluctuations when used with high power USB dongles such as USB modems. To solve this issue, my implementations rely on a high current Dlink five port USB hub to supply power to both the SBC and the USB modems.

I also found RasPBX to be overly bloated and chose to install Raspbian, strip it down to remove all X components then install Asterisk 1.8, FreePBX 2.11 and chan_dongle.

On the GSM end I went with Vodafone branded Huawei K3565 modems for use in Europe and Huawei E181 for the Americas.