Cannot Call in

I have a simple Asterisk box using 1.4. It works fine. I just have a few extensions and two call out trunks. Recently I had a IP Communications call-in number. But it’s not working. when I call this number my extension rings. As soon as I pick up the phone, the call was dropped. I used FreePBX 2.3 to configure my PBX. I didn’t edit conf files directly. I just have a simple incoming route which goes to an extension. In General settings I set “Allow Anonymous Inbound SIP Calls” to YES.

Can any one give me some hint what could be wrong?

Thank you very much.