How to setup 7971 phone from home over broadband using CM public IP ?

Hi, I tried to setup Cisco 7971 phone at my home to be connected to the office 24/7, I have public IP address to connect to the CM, and in 7971 phone I put the Public IP of CM to pull TFTP config! I can see on the phone screen the extension, with a red cross mark on the line, but there is no tone and line is not working! Phone trying to get registered and restarting constantly. I added this phone in the CCM as a SIP phone. but still no joy!
Any help will be appreciated.

Thanks