[Freeswitch-users] Caller Authentication

Guys, This question might be a bit off-topic, so please bear with me as i could hardly find a better place to solicit some help. I suspect caller authentication can be challenging in the broader context of VoIP/SIP. Now, if we restrict the set of calls to those coming from the GSM network, is there a way to tell if the Caller Id is coming from the right source? It doesn't need to be a bulletproofed solution. For instance, if i can already say that a given call is certainly coming from a GSM network (and that the call metadata weren't tampered with), i could maybe rely on the GSM authentication mechanism. Any idea? Max.