Inbound calls not working after some time

I have a problem that I need help troubleshooting. I have an FreePBX installation that has been working with no problems until recently I stopped receiving inbound calls. Restart always fixes that and it always works for few days, sometimes even weeks. And then it stops. I have two problems with that:

  1. Is it possible to regularly check that inbound calls are working other that actually make a call?
  2. How can I resolve this? When it´s not working, I just get a busy tone, nothing in PBX. It looks like the SIP provider was disconnected, but I don´t know where to start looking.