Forwarded Calls to Mobile/Cell Drops

Hi All,

I have a ring group that includes the office number and a few mobile phones. When the number is called all the phones ring perfectly. When answering using a softphone or physical phone within the office it work fine. Soon as i answer on my mobile it disconnects.

Anyone come across this problem or have any idea how to fix this?

Also, not sure if they will be related, But if no one answers it goes to voicemail. But before you can leave a voicemail the call drops again. Any help on this would be great as well!