In message <Iishk.15991$mh5.10641@nlpi067.nbdc.sbc.com> DTC
<DTC@no_spam_nothing_here.foob> wrote:

>It still works for their ATT phones, but the manger upgraded
>to an iPhone and the ATT VM no longer recognizes his number.
>After you input the [cellphone number], is says its not a valid
>number, but his iPhone can still call his VM.
>
>Any clues?

Due to visual voicemail, the voicemail systems are independent. If you
have another phone, you can swap SIMs to pry the numbers out.

In short, what you need to do is pull the iPhone's SIM card, pop it into
another GSM phone.

Press and hold "1" until
voicemail dials, then hang up, and check the recent calls.

Most phones will reveal the number, although I did have one (Sony w810i?
maybe, not sure which phone it was anymore) that just showed
"voicemail", in that case, adding it to the address book revealed the
number.

On Rogers' network in Canada, the iPhone visual voicemail system is
+1.6478029327 -- I realize that doesn't help you, but it does confirm
that this technique is possible.