Call forwarding to External mobile number when Number busy or unanswered !!

Please I need help. I want to forward all inbound calls to external mobile numbers when busy or unanswered. I did a config on an extension with regards to "eternal mobile number". If I call the extension(160) internally its able to route my calls to the external number. But one calls from an external number to the inbound, the call drops upon a voice prompt saying the routing initiating is taken place. Please what can I do resolve this ?