[Serusers] No incoming calls, why..

Hello, I have installed cisco phone (7960) and another SIP phone in the local network which is behind firewall. I'm able to register both the phones with different usernames of iptel.org I have tested both the phones seperately with echo tester. Thats working. But, I am not able to make call from cisco phone to my sip phone and vice versa. I am not getting any response for the invite. Not even 100 trying. Is it because both the phones are working behind nat/firewall or some other reason. can anybody in the public ip try san(a)iptel.org <mailto:san@iptel.org> or dheeraj(a)iptel.org <mailto:dheeraj@iptel.org> ..? Kind Regards, Santhosh.S