[Serusers] usernames but no pass

hi i think is got a unique problem. i have a phone system that until the final software package is developed will only allow a user name but no pass and i need to make sure that the server isn't taken for advantage so only a few very long and complex user names will be used. i tried adding a user then going into the mysql and deleting the pass but it seems ser doesn't even get that far if the client doesn't send a pass. so to recap i need a user without a pass but only specified user names to work. also some guy hinted about gateways though I'm not sure he knew what he was on about, so if any of you know that would be good and if their are any documents in ti as well that you could point me to that would be good also. thanks Alex Wood independent contractor