ICMP port unreachable messages from asterisk server

I am running a small asterisk server at my house. I have a single IAX trunk to a VOIP DID service, and several SIP softphones on different computers, each configured as an extension. However, only one of my computers can register. The timeout when registering. I ran wireshark and saw ICMP port unreachable messages in reply to the SIP registration packets.
I do not understand why one of my computers can connect to port 5060 (SIP) but the rest cannot. I am not running a firewall.

When you say you created extensions did you create an individual account for each phone in sip.conf (or users.conf) ? Where did you run the wireshark, On the phones end or on asterisk ? Anything come up when you run sip debug from the asterisk CLI ?

Yes, I created accounts for each phone in sip.conf

Both. Nothing shows up at the asterisk end. On the phone end, I can see the outgoing SIP registration packets followed by ICMP Port unreachable packets returning from the Asterisk server box.

Not a thing.