I purchased a Nexus S in the US because I was moving to China. After arriving in China, I received my China Mobile Sim card, but every time I try to call that number, the phone is busy. I can place telephone calls, and send and receive text messages. The SIM card works for receiving calls on other phones (except my Galaxy Tab) which is experiencing the same problem.

My thought is that there might be a running service that is tying up the line, but I've stopped a bunch of them, and that has not solved the problem.

Any ideas?