I do not always receive incoming calls on 3G

Your 3G operator seems to filter UDP connections, that's why You can't receive incoming call. To support Your connection, it is neccessary to set Zadarma program to be more agressive.

In order to make it, change Your account to Expert mode (to change the account, press menu "Choose master" and choose Expert). Again go to account editing, several new parametres for setting are available now. Try this:

  • Registration time (sec): 184
  • Keep-alive: 100
  • Depending on Your device, You can activate CPU Clock, for that purpose change to «Expert» in section of common settings.