Perhaps he was confused and thought you asked for the unlock code for a different carrier locked VP (the guy selling the unlock codes I mean). (ie the unlock code for a tmo locked one for example)
I'm assuming the ones that give actual codes (and not have you use their unlocking software) are simply pretending to be carriers/users and getting the official unlock codes. Each carrier has it's own database/section on the manufacturer's database.
If you only need 900/1800, are you only using EDGE/2G?
XDA Wiki Moderator
Nexus 4: Stock JDQ39 (4.2.2) Nexus 7 Sucessor: Waiting for it
iTouch 5G: Stock 10B146 (6.1.2) Venue Pro: Stock 219-33 (7.10.8862.144)