I'm afraid I haven't gotten round to playing with Asterisk yet, it's not included in the DD-WRT build for my (cheap and nasty Virgin Media freebie) DIR-615, so the SIP is all from third-party providers here. As a statement of general principle, I haven't been able to get any form of VoIP to work over 3G on any of the UK mobile networks - I'm not sure if that's a network restriction or a purely technological one. SIP works fine over my home WiFi, and not at all over very much else.