Does anyone know how to set the multi-purpose buttons on the phone to ring a specific extension? They are currently set but are no longer accurate (people have changed offices, phones have moved around, etc)

I have the 12 button phone (no display) - every manual I find online makes reference to the multipurpose keys but does not lay out how to set or change them.

Thanks in advance, this has been a thorn in my side and since I'm the IT person everyone is hounding me about it.