Mobile extension will work with analog but is not recommended.

Receiving calls
My desk phone fix call forward is set as both ring to the mobile extension 164.
Extension 164 fix call forward is set as no answer to InMails pilot number.
(When mobile extension is setup the CPU will look at program 13-04.

When a call is transferred to my desk phone by a co worker or auto attendant the call rings both my desk phone and cell, if no answer goes to my desk phones voicemail.
Extension to extension calls also ring my desk and cell

The only bug I haven't figure out (not a priority) is when someone calls my DID the call only rings my desk phone. I see the call come in and at the same time the last channel lights up and drops.

We get old too soon, smart too late