Check the IP that your 3G connection gives you. If it is a private address such as 10.* you are NATted anyway.
In which case you can try the instructions
regarding connecting from inside a NATted network, which happen to use a different domain name and port number.
But yes many mobile operators routinely block SIP ports - this is not anything to do with Voiptalk nor their responsibility to workaround. One could argue that your mobile provider is not really providing Internet access but a filtered subset based on things they don't see as a threat to their business model.
An operator I know for sure does not block SIP is Three, but you may need to alter some preferences regarding how often the SIP client re-registers to continue to receive calls. Making calls should be ok.
Also, it's a good idea to direct "calls while offilne" > voicemail > email so that if your SIP app becomes unregistered you'll receive any voicemail promptly anyway.
prompt $P - Invalid drive specification - Abort, Retry, Fail? $G
prlzx on n e w n e t: ADSL2+ / 21CN at 2.5Mbps / 800k