I'm trying to figure out the exact way they do this.

For AT&T and T-Mobile, you can dial a special GSM code on your phone and get your current voicemail backdoor number. You (or others) can then call that number, navigate through a menu of options, and leave a voicemail in your mailbox without calling your phone. I can't imagine GotVoice is doing this since there are tons of different backdoor numbers and they're subject to changes. Also, the menu systems of the different voicemail systems even on the same carrier can be different, so automating that seems impractical.

Even if they were using specific backdoor numbers for each user, that doesn't explain how they're able to leave messages for Verizon or Sprint that, to my knowledge, don't even have these backdoor numbers.

There's also SlyDial (www.slydial.com) which seems like they place two calls at the exact same time to get one to go to voicemail and after immediately drop the other line (sometimes the called phone half-rings). GotVoice's system doesn't cause the phone to half-ring, so they've gotta be doing something differently.

GotVoice claims they can do this regardless of carrier: "GotVoice has a unique technology platform that navigates voicemail systems and bypasses the normal call route, which enables the audio content to be dropped directly into the voicemail system, regardless of the carrier."

I'm trying to replicate this functionality on an Asterisk system for a project I'm working on.

Could some telephony guru chime in and tell me what could possibly be going on here? I'd really appreciate it. Thanks.