To receive calls, a user-agent (UA) must be registered (which is not necessary for placing calls). To do this the UA sends a REGISTER SIP packet to the node and provides the current IP address and port. The proxy keeps this information and sends INVITE there upon an incoming call.
There are two main reasons why incoming calls may fail.
1. Disabled Keep-Alives in UA. This ensures that the router will not 'forget' about the device and will not close the NAT hole, so the registration IP:port information will remain valid.
You can see this in the IP phone settings and CPE profile as follows:
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2. If the router has SIP ALG enabled, it must be turned off.
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