What is the output of “sip set debug on” when you attempt to place a call? (If it is coming in over SIP). If there is nothing then it is likely that the traffic is not reaching Asterisk and you will need to look at the network itself.
What is the network topology and are any firewalls or NAT in place? Can the other side get to the Asterisk server? What are you attempting to call from?
That is Asterisk sending a SIP message to determine if a remote device is still there. I’m not really sure I have anything else to offer, the calls are not getting to Asterisk.
Could it be that, the IP phones I’m using have static IPs which I guess does not need to register with sip. Because I haven’t registered the IP phones with sip.