[Serusers] Call type processing Section problem?

Hi. Till now I was happy using SER handling communications between UAs using nathelper and RPT proxy. Now I changed the ser.cfg script to the one with PSTN support and none of the UAs get registered. I am getting a 403 Forbidden. Analyzing the script I understood that this 403 was coming from the Call Type Processing Section so I changed if (!is_uri_host_local()) { if (is_from_local() || allow_trusted()) { and so on to if (uri!=myself) and now the UAs are registering again. But of course I can't make calls. So I would like to know how this< is_uri_host_local and is_from _local> work and what should I do to make them allow my UAs to register. Thanks in advance Juan Ferrari