There is a bug for IAX registration in Asterisk 1.8.1.1, in fact, it has been existing since Asterisk 1.8 release. Someone had reported errors when using Zoiper for testing and registration.
A patch has been released to remedy this bug but I don't know how to apply to my existing system.
For informatin, IAX registration on the side of IP01 with Asterisk 1.4 is fine without problem. However, we may encounter registration problem when used in connection with Asterisk 1.8 from time to time, i.e., connection and disconnection happens intermittently without reasons.
I do not have any problem when registered with Zoiper. Indeed Zoiper has problem in the version from 2.35 onwards. I have reported to them to fix but still not ok.