Call forward for SIP one extn to mobile number - Not working

Hi ,

I am having asterix pbx -> free pbx -> i am new to this asterix (Recently deployed)
Cisco ip phone 6291 & SNOM 70 series

when i am trying to forward the call from one extension to mobile number in the ip phone , 
Call is set to forward and display in the ip phone . 
1. When i am trying from the exten to another extension internally - call is receiving to the mobile number forward 
2. But when i am trying from outside to the extension number - ring goes up to 3 times and then hung up the call .

Kindly suggest and guide me how to resolve - what are the things need to check