Can make outgoing calls, can't receive.

I have 4 desk phones. I can make outgoing calls to mobile and landline numbers.

The problem I have is when I try and call my main number or a direct number I am prompted with the greeting but none of the desk phones ring.

I get the "Please hold while i try and connect you" message and music.

Then after about 30 seconds it goes to voicemail.

What have I done wrong?