I don't always receive incoming calls on 3G
Your 3G operator might filter UDP connections, that's why you can't receive incoming call. To fix it, you need to make some changes to your app default settings to make it "more aggressive".
Change your account to "Expert" mode (to do that, use menu "Choose master" and choose Expert). Now go to account editing, several new parametres for setting are available now. Try this:
- Registration time (sec): 184
- Keep-alive: 100
For some devices, you can also activate CPU Clock after changing to «Expert».