I unlocked my iphone 3g using ultrasown (3.0 software) and mailed it to my cousin in the UK to use on o2.

I tested it here in the states before sending it and it was working perfectly.

When he put his sim in he is able to make calls but he cannot receive any calls.

Is this a known issue with unlocked 3g's on 3.0 and o2 sims?
Are there any known fixes for this issue?

Thanks in advance for any help guys.