I can confirm it is definitely working with pjsip. Besides the logs as david mentioned can you also post the System identity, SIP interface and Registration settings of the phones web gui. If I remember correctly I had the same issue with the CP200 but it only was a small setting on the phone. I also have access to one CP200 right now and can compare.
[Edit]
Did you have any value in Terminal Name field? I think that was my problem in the past. This field needs to be empty. But I am not 100 percent sure.