If you want and it sounds like you do, to send each users name with their individual ext.
You will need to verify the digits your vendor is receiving from you and have them assiociate a name to each ext.
there, ofcourse, is a cost for this and the upkeep of names to extensions, depending on turn over, can be very demanding.
you can in your "mask" send out the user extension, without a name, to give the caller a number to call back on to reach the individual user.