We use something like this:

        if ((uri =~ "^sip:90[0-9]{6}@.*") && (!avp_check("$avp(s:perms)","re/9/g"))) {
                xlog("$ru : No permissions to call 90X numbers ($ci)");
                sl_send_reply("403", "No permissions to call 90XXXXXX");
                exit;
        }