How to Troubleshoot Asterisk VOIP PBX Phone System that does not Accept Incoming Calls

1. Make sure that only 1 Server or Virtual machine is binded to 5060 port, otherwise the Firewall might be conflicting on forwarding the packets to the correct Virtual Machine.

2. Make sure that the Asterisk PJSIP.conf configuration file is configured correctly to communicate with the Phone Trunk Server.

3. Make sure the Asterisk Server or Virtual Machine has actually registered with the Remote Trunk Server your VOIP Service Provider.

4. Make sure that the Virtual machine has internet and that the DNS is actually resolving.
5. If the incoming phone calls does not reach the phones:
    1. Make sure the sip.conf file has an IP address of the Trunk Server
     2. Make sure the Server can ping the outside Hosts, sometimes the DNS might be blocked.
          - restart dhcp service
                sudo dhclient -r
           - reload dhclient
                sudo dhclient
     3. Turn debug on to see the logs, make sure you set debug off again when troubleshooting is over
            sip set debug on