If a call connects, but two-way audio is not achieved, then the firewall is often to blame; at the end where no audio is being received. Sometimes such a call will time-out after say 30 seconds.

Some routers employ a SIP ALG Helper, which is supposed to help with VoIP calling, but is generally not helpful in our experience. You should disable SIP ALG in your router settings.

See also: Which ports should be open for In:Quality services?