phone registration in srst mode

hedhrts

Has anyone come across documentation on how to allow only specific phones to register in srst mode? I'm sure I read about this feature a while ago, made a mental note of it (I'll write it down next time). I've been looking through all sorts of docs but can't seem to find this feature again. I would appreciate a push in the right direction if anyone if familier with this.

I saw Luckycharms "MGCP Fail-over Registration Restriction" post and had already come up with this solution, but I'm sure I came across how to do this in the gateway.