SIP Port number problem

I have the IncrediblePBX distribution of FreePBX running in Virtualbox and am having a problem registering the FXO port of my grandstream HT-503. The HT503 wants FXO on port 5062 and FXS on port 5060. (FXS is connected directly to callcentric and works fine independent of my new PBX.) In FreePBX, for the extension I am using, I set the port to 5062. The trunk peer details defines port=5062 and the invite is ext:[email protected]:5062. The channel sip info report shows the host with the right port# and the extension port correct, but the status is unreachable. I checked firewalls and finally ran wireshark to see what was on the line. It shows a SIP request followed by Destination unreachable. The SIP request shows source (HT-503) port 34727 and destination (FreePBX) port 5060. I checked and the HT503 FXO is set to 5062. Since the HT503 uses port 5060 for the FXS port, I disabled FXS and still get the same info on wireshark (just a different source port). While I had FXS disabled, I changed all the ports (FreePBX and HT503) to 5060. Wireshark still saw everything on port 5060, but still alternates request/unreachable. I think I am chasing two problems - setting the port to 5062 and finding out why the request is unreachable. Any ideas?

Maybe I can get some help with the sequence of events that is supposed to happen with a SIP registration. What I think I see on wireshark is that the HT503 sends a SIP request to the PBX (from port=58379, to port =5060) using UDP. Then the PBX replies with a port unreachable status using ICMP (the reply even lists the MAC address of the source and destination). However, the details of the SIP request has everything I set up in FreePBX. Am I reading the wireshark report wrong? Is that the correct sequence?