It depends on your configuration.
If you are establishing the SIP session over TCP, make sure you open 5060 on TCP rather than UDP and vice versa.
You should also open ports 10000-20000 UDP for the RTP stream.
Again, this is all depending on how you have your system configured.
You can restrict those ports to specific providers. But many sip trunk providers have media servers all over the place and it is difficult to be all inclusive.
Good luck!