Do you have Mobile Twinning, known as Dynamic Extension Express on your 5000?
When you first log into Database Programming do you have the option of Users, between System and Voice Processor?
If you have Users as an option create a User matching your 53xx phone and within here set it up so his mobile is twinned with his desk phone. Whenever calls are routed to his deskphone his mobile will ring as well.
The call forwarding hasn't been working as you can't forward a call that arrived at your deskphone via a Hunt Group.
If you haven't got a new enough system or the license is missing you may have a bit of a battle on your hands to get this to work.
Are there other members of the hunt group, or is there just this one phone in there?