Is the phone registered to CUCM or CME ?
It is pretty simple for CUCM - set the call forward no answer setting on the Directory number page to the mobile number ( make sure to prefix 9 or any other prefix to match a Route pattern ) on CUCM.
For CME , you can use the call forward noan
You can find more details by searching on supportforums or google.
Alternatively to Manish's great answer (+5) you can invoke single number reach (mobile connect) feature if you are on a version that supports it.
That's a nice alternative from Chris [+5].
Configuring SNR on CUCM
Configuring SNR on CME
Uncheck he Voicemail boxes in he picture. As long as the boxes are checked the desinaion number will be ignored and the forwarded call will always go o voicemail.
Are you rying to call from an external phone to trigger CFNA?
Is there a route pattern configured which starts with 99 or 9. , can you share the exact route pattern that you wish to use?
That's because you have left the 'Forward No Answer Internal' field blank, if you populate that one, even the internal calls would be forwarded.