Voice spam is impractical (Score 3, Informative)

The way SIP works makes voice spam impractical. Basically, a call is set up in two steps. 1) The calling party sends an INVITE message to your provider's PBX / main server / whatever. This would be vonage, or whoever your VOIP provider is. This 'call' connects, and an audio path is established between your provider and the calling party. From the caller's perspective, he has a live, answered, call at this point. 2) your provider sends an INVITE message to your phone. This establishes an audio path from your phone to the carrier. At this stage, the carrier either connects the two audio streams internally, or can use another pair of INVITE messages to direct the audio streams of the two phones to each other. There's no way for the calling party to identify when that second audio stream has been established; from their perspective, the call exists as soon as the provider accepts the initial INVITE message. Obviously, you could start playing audio at that stage, but there's no guarentee someone's actually on the other end of the line. If you're doing a recorded audio play, you're faced with either loosing part of the message, or playing dead air for a while. The only way around this is to dial the direct SIP extension of the customer's phone, but you need know their userext (which is different than their actual phone number) and the IP address of the user's phone, which is highly unlikely since the end user doesn't even have those bits of information (usually) Furthermore, filtering is easy. An INVITE message has to specify a valid IP for the audio stream to be set up. It's trivial to simply block INVITE's from certain IP's in software, if your carrier / phone supports that. Spoofing an IP at this stage is impossible, since that would just prevent the RTP stream from working, and it also makes it easy to figure out who's actually calling you, since you have the IP of the server the audio is coming from. (assuming your provider did the reinvite bit, which virtually all SIP implementations do) That's totally ignoring the much higher bandwidth requirements of transmitting that many audio streams and associated problems with that.