I've installed asterisk on my Raspberry Pi but haven't bothered setting it up yet. I plan to soon, I have the original one with only 512kb of ram. Installing it is as easy as running apt-get install asterisk on the RasPI distro (it's obviously debian). It installs and runs fine, so I see no reason why it wouldn't work. I've also read where others are running it without issue.
I'll have to look into the GSM devices. To further theorize, check into fake cell towers to intercept calls, the same hardware should work. There should be some vids about it. There was a talk at defcon a couple years or so ago. I know there's at least a power point document with the details. If I run across it and remember, I'll drop a link to it in here.