We put in a new IP phone system recently that does something similar. Previously we had to get the MAC address of the phone and advise the administrators (I was not one of the trusted few) to associate that MAC with the desired extension number.
The new system now lets you configure a phone without a MAC. You go and plug in the phone, lift the handset and get a voice advising you that "this is not a provisioned phone". You have to enter a password and then the desired extension of the phone. The system then does the associations itself. Much better way of doing it.