I’m supposing you want to allow 15XX users only to call 15XX extensions, so what you have to do is create a context for the 15XX group that bans any call to 10XX, 11XX, etc, except 15XX. It would be as follow:
[from-group-15xx]
exten => _1[012346789]XX,1,playback(ss-noservice)
exten => _1[012346789]XX,n,Hangup()
include => from-internal
In this way you can configure a PBX hosting multiple companies without mixing them. The companies wouldn’t be able to make calls between them, although they could receive external calls, make outbound calls and make internal calls only to the extensions you have assigned to them.