I am sure I was told on a course that the embeded voicemail has to have mailboxes with all the same number of digits. however I have a customer who is asking to retain an existing dial plan from a panasonic which has a mix of 3 digit extension numbers and 4 digit extension numbers. Is this going to cause me problems on the voicemail?

I'm curious about the same thing - I think we ran into issues with menus.

Quick answer is yes. You have to pick one or the other for the VM.