Possibly, this thread will help:
If not, try setting Expiration to 120 or Qualify Frequency to 30.
If no luck, report whether the trunk still shows as registered after incoming calls stop working, as well as whether anything appears in the Asterisk log on a failed call.
Also, do a test by making an outgoing call, then immediately (within 30 seconds) trying incoming. If that works, post router/firewall make/model and any VoIP related settings.