Ok, I think I solved the issue.
I can just add the phone (CIPC in my case) using the mac address of the phone as that is the name of the cnf file that the phone is asking for over tftp.
The only restriction is that the mac-address has to be hex and 12 digits long.
(CME does not do a reverse arp to validate that the phone is on the same subnet)
However, when I register CIPC with CM I can give any string and it does not have to be in the range A-F.