I have Issabel working just fine with Zoiper and several other softphones. Tcpdump should be able to show you what is happening.
If anything is behind NAT that could also be a problem.
You have not provided any information other than not registering which could be just about anything. Do you see the phone attempting to make a connection while you are watching asterisk console?
asterisk -vvvr
If you don't see anything happening while the phone is trying to register then it's probably a network related problem.
Can you ping the server? Do you see any SIP traffic? Are you sure you have the correct IP address configured in your phone?
