[Non Alfa] Benz Head Unit Coding

[Citroënbender]

Trying to help out a friend whose iPhail (a 7s) won't pair with the head unit (Comand NTG 2.0)  in an older A-class. It's the type with a separate little Bluetooth gizmo clipped into the centre console.

It looks like physically the later head unit with integrated Bluetooth (Comand NTG 2.5) will fit, but there is copious reference on the net to coding a head unit in. Much of the chitchat I've read seems secondhand, or uninformed or otherwise flaky! What I'm trying to get to the bottom of, is whether the head unit simply unlocks with a PIN obtained by means fair or foul, or if it has to have the VIN changed to reflect its new "home" vehicle, or if there's some other configuration routine...

I haven't tried pairing it with an older Android phone but that's something else to try.