Hi Del,
In Ring/Hunt groups the AGI builds the group dial statement "on the fly" from the database and then issues it directly it to asterisk. Call Forward processing is ignored during this process so Aliasing won't do what you want.
If only your desk phone and mobile are involved then you might just point the trunk directly at your desk phone and use *22* to transfer to your mobile on "busy or no-answer". I have *21* to my mobile setup on one of the programmable keys in my desk phone. This means I can just press a single key when I leave the office and my calls will be forwarded to my mobile.
I believe it's also possible to transfer calls to an alias using *22*. So you might want to transfer to an Alias which fires a hunt/ring group of its own. For example, maybe ring your desk phone first then hand off to an alias which calls your home phone and your mobile. Stuff like that.
Kind Regards
Selintra