Security! Call Forwarding Hack?

Yes, a phone number can refuse incoming calls. When you access it via Voice Mail, it is not the same access. I suggest you talk to ATT. They also have access to internal WLNP databases. The number is a 'peering' number, not a number that can be called directly. Typically, such numbers are used when a subscriber wants to access their VM, the VM system is 'dialed' on your behalf by the SP's internal systems (a HLR/HSS in most cases), but for security reasons, such numbers are allocated from a pool that disallows incoming calls.

The other use for such numbers is allow data connections from wireless phones where the client (your iPhone) behaves like a modem and dials a point-to-point number to setup up a connection with a bank of modems kept by the SP within their infrastructure. The SP system acts like a data gateway for your device and anchors your data connection.

The Wireless system is a spaghetti of old analog technology on the SP side which a subscriber usually is never exposed to.

What you're describing is sometimes called No Answer/Busy Transfer. It's not simple call forwarding. It would have to be done through the carrier. At least with Verizon, it doesn't require any PIN be sent to the phone. I just dial a * followed by a two-digit code and the number. It's how I use Google Voice for my voicemail instead of my carrier. Anyone who had access to your phone could have done set this up.

On Verizon, to enable No Answer/Busy Transfer, also called Conditional Call Forwarding, I dial *725555555555 (where the fives are the number I'm forwarding to). I then get confirmation beeps. I don't know how it works with ATT. You'll need to ask them. You should also be able to get them to remove the feature from your account if you want.

Call forwarding on the phone itself is not, to the best of my understanding, conditional call forwarding. However, I don't have an ATT phone on which to test this.

You have a Voice Mailbox that has not been set up yet. You should ask ATT about the details but I suggest you start here - https://www.att.com/esupport/article.jsp?sid=52653&cv=820&_requestid=759186#fbid =_-Wc6-2QgQ3

Just FYI, The voicemail bot that speaks your number to the caller is normal, that's the Default greeting...

Also, where's the proof in forwarding to a different number?
How did you find this out?

Why are you mentioning broadband.com?

Have you had At&t reset your voicemail?

This may be a shot in the dark, but I know Verizon/Sprint phones actually have two phone numbers... I don't know if At&t uses two or not... but if they do, that's probably what you're seeing.

Or someone, as a joke or for other reasons, enabled it from your phone. Or, it was a weird system glitch. Or there is a way to enable conditional call forwarding from a web page and someone typed your number in by mistake. Conditional call forwarding is a standard account feature for cell phones.

Computers aren't perfect, telecom software sometimes has strange bugs, and adding users to the mix doesn't add up to make for a 100% reliable system.

I think you went to the extreme Avid... Maybe At&t goofed and a rep fudged up someone else's number getting CCF and yours happened to be the typo.

If strange things keep happening, change your phone number...

To support WLNP, ported numbers are required to stay alive for aging purposes in a SP's system. If your number was assigned to you but was previously ported by someone else into ATT, the association of your number with the previous provider is meant to to expire in 24-48 hours per FCC rules. I have seen glitches where such expiration does not happen at all because the previous SP keeps the association between it and other systems, like VM.

If you have call detail on your ATT bill, you can see prior history (required by law for ATT to keep) and Call Forwarding transactions are recorded including the *72/*73 transactions.

In the CDMA world, there is a MSISDN associated with a MN so you have a 'peering' number that you can call. Very old legacy of roaming.

Is it possible that you have applications that can make calls apart from your regular iPhone dialer, like Talk-a-Tone, Skype, ObiHai, etc.?