You can use personal ring groups (PRG) or multi device groups (MDG)
You will need one user programmed with the mobile number filled in (need a external hotdesk lic for this (EHDU))
put the desk phone user and the EHDU user in a PRG or MDG
make sure they are shown as "present" in the group
In the class of service (COS) for the EHDU you can choose to have the EHDU permantly logged in
Called something like "external hotdesk user perm login"
If you dont set this option in the COS they you need to ring in from the mobile to the DISA number and "log in"
There are a few settings so if you need more info let me know, i may right a guide if i can be bothered.