Linphone has a free Android client (on the app store) and provides a free sip service. Depending on the codec, bandwidth is between 20-100Kbps (from memory). My maths is probably incorrect but 2GB of data should give you more than 200 hours of talk time on a low bandwidth codec.
I had linphone on my android for a while and it worked ok but I had the usual SIP behind firewalls problem when trying to use it over wifi so that is something to be aware of.
I would be interested to hear how you go with it as I didn't really have anyone to call via SIP so uninstalled it in the end. Asterisk is fairly easy to install, configuration and security can be another issue, especially once you start dealing with firewalls and routers. SIP can be a finicky protocol (IMHO). I'd just go for using the free linphone service personally. Speaking of SIP services, does anyone know of any providers of federation or interconnection between SIP providers? It seems like most SIP providers still don't want to talk to each other.
- Lauchlin