[Users] Checking for Digit Length

Hello all, I haven't been using OpenSER long so forgive me if this is a silly question. We are currently using OpenSER 1.0.1 (no TLS) running on CentOS 4.2 and 4.3Linux, if that helps. Basically I am trying to find out a way of checking for minimum and maximum digit length dialled by our users, in order to filter some of the traffic through our OpenSER server. Dialling rules aside, is there a way? Perhaps something involving checking the length of the Request URI in an INVITE message and then possibly sending a "404 Not Found" or similar message back? Is there a particular module I should be looking at too, or some generic functions that can do this? Your thoughts and comments very welcome. Thanks in advance. Max.