No incoming calls until you make an outgoing one,?

Question

Asterisk

There is a sip trunk. No incoming calls, but outgoing calls work. And what is most interesting is when you make one outgoing call, within a couple of minutes, incoming calls start working. After again the calls do not come

Answer 1

We decided it was the modem's firewall.

Answer 2

The problem is NAT, configure the router correctly.