This thread is a little over a month old, but its topic has been on my mind tonight, and so I made this post.
When I first read about Slydial, I immediately presumed that it simply called the voicemail access number (pilot number) for that cellular exchange or thousand block, and either DTMF'ed the cell phone number or spoofed the appropriate
rDNIS data through SS7 (in SIP it's in the Diversion header). When spoofing the Redirecting Number (RGN) in the rDNIS to a specific cell phone number, the cellular voicemail system will route the call to the voicemail box of that particular cell phone. This data is normally sent and used when you call someone's cell and get redirected to voicemail, to which the voicemail system gives you the appropriate voicemail box based on that RGN it received. Compare this to normal
DNIS, when your call is automatically routed to a specific extension or IVR on a PBX when going over a PRI line, based on the port/channel reached when dialing a specific telephone number among many that would otherwise reach the same destination.
You really don't need Slydial to leave someone a voicemail without calling their phone. You simply find the pilot number and enter their number to leave a message. You can also call another number on the same carrier and wait to reach their voicemail, then hit * or # to backup and enter another number. This only works with certain providers such as Nextel, though their pilot numbers are easy to find anyway, because they always end with MAIL (NPA-NXX-6245).
Anyway, that aside, it turns out that my own theory was wrong, because this is
not how Slydial gets you directly to the voicemail. It's much simpler: Slydial places 2 or 3 simultaneous calls to the cell phone number, and then connects you to the first call that supervises (gets answered). This works because there is a limited number of ports for a single cell phone line, and when 2 to 3 calls come in at the same time, one of them will be unable to reach the party while the other(s) tie up the line, and therefore get redirected to voicemail. This explains why the site claims that the party's phone will sometimes "emit an abbreviated 'half ring,'" since there could be a slight delay in the VoIP service disconnecting the other call(s) before the phone can ring.
I haven't confirmed all of these details yet, but there's a way to find out for sure. Forward a cell phone to another number that can receive multiple calls. This could be a conference bridge that lets you detect the number of participants, or a VoIP DID number where you can monitor the call traffic. Being able to detect the number of incoming, non-supervising calls (when they're still ringing) is probably the best bet. Then have Slydial call your cell phone and attempt to reach voicemail. If this theory is correct, 2 or 3 incoming calls should be detected at once to the forwarded destination. You may want to make sure that at least 3 forwarding ports are available through your cell phone's network before this experiment is conducted.
For further confirmation, you could have Slydial attempt to reach the voicemail of a cell phone number that was ported from another carrier, since the exchange or thousand block would not accurately determine the correct carrier via publicly available LERG data (
http://www.telcodata.us). If the voicemail is not reached, then this would also disprove the first theory of Slydial calling the voicemail access number (pilot number).
I also wonder why Slydial claims it does not work on prepaid cell phones. I know of a few prepaid carriers that simply resell lines on major cellular networks like Sprint and Verizon. It just doesn't make sense to me yet. I'll post again later when I learn more.
