atsak read again, he does have the ring group set with the # on the end of it (hash).
Check the person's follow me -- if it is disabled (meaning it rings only the extension, this would be why. Also make sure the number to be dialed, has a hash mark too and a correct trunk to get out.
If the person always want thecell to ring while in the office as well, then enable Follow-Me, put his desk extension on first line (no hash on it) and his cell on the second line with a hash. Select 10 or 15 as initial ring time, and ringallv2 as strategy. Total ring timme at 60 (gives time for cellphone to ring.
trupsalms, the more details you give us, the more we can help you (raspi, centos, debian, issabel, wazo) they all have different ways of doing things. What I just gave you is Incredible with Asterisk 13/FreePbx13, you hope it works for you.