Cisco 7940 not registering

Hi,

I have a weird problem on cisco 7940 that is not registering to outside world. My setup is as below:

1. Elastix server on private IP.

2. All cisco phones connect with elastix server and gets registered with internal extensions.

3. While assigning sip account on line 2 it's not getting registered. I have configured particular SIPXXXXXXX.cnf file with proxy address and nat settings but still it's not registering.

Could anyone help me solve this issue as i might have missed some configs here?

Thanks,

Kijush