asterisk users - Sep 2016 - Customizing the messages for voice mail

I know that users can create their own messages but I can't see how to
change the extension number.  Here is my scenario.

Users extensions can be any string.  Sometimes it is an actual phone
number but often it is their user ID.  Let's say that two of them are
foo and foo3.

I also provide virtual PBX extensions that accept an extension and
direct the call to a device.  A phone can be in more than one PBX so
calling 4155551111 might assign extension 200 to foo and 201 to foo3.
Calling 4165552222 might assign 210 to foo and 220 to foo3.

All of this works great until a call goes to voice mail.  Now the foo
extension says "The person at extension <short silence> is not
available"...  The message for foo3, which might be extension 201 or 220
in the above example depending on which PBX was called will say "The
person at extension 3 is not available..."

So my question is this.  Is there some way in the dialplan to set the
number that is used for that message?  Is there a variable that can be
set before doing a GoTo to foo or foo3?

Thanks.

-- 
D'Arcy J.M. Cain
System Administrator, Vex.Net
http://www.Vex.Net/ IM:darcy at Vex.Net
VoIP: sip:darcy at Vex.Net