For an external call to be forwarded, the trunk cannot simply be in a ring group, as technically the phone isn't ringing, the group is. You have to assign the trunk as a DIL to that extension (22-07) and change the trunk from Normal to DIL for the specific service mode in 22-02.
Then you have to see if that trunk rings more than one phone. If so, you can put a Virtual Extension key of the primary phone on the others in 15-07, then assign that key to ring in 15-09. All of the phones will ring, and the call will follow the forwarding of the primary extension.