asterisk users - Sep 2004 - stale voicemail messages / greeting

I'm using Asterisk to read voicemail users out of a SQL database.  I am
assigning users real phone numbers as their voicemail box.  The problem is
that if I re-assign a phone number (say, 972-245-0001), the new user is
stuck with the old user's greeting and saved messages.  What is the best way
to resolve this?

I don't want to use unique mailbox ids because my dialplan looks like this
in the incoming DID context

[incomingdids]
exten => _972245XXXX,1,setvar(boxnum=${EXTEN})
exten => _972245XXXX,2,VoiceMail(u${EXTEN})
exten => _972245XXXX,3,Hangup

exten => a,1,VoiceMailMain(${boxnum})
exten => a,2,Hangup