On any ATT cellphone account, you can learn the account's voicemail deposit phone number by entering *#61# and pressing CALL or SEND. A voicemail deposit number is useful because you can dial it to leave a voicemail for the customer without ringing their phone. An iPhone user in the San Francisco Bay Area might have one of these: 253.709.4017 253.709.4072 916.952.9980 916.835.1098 678.763.9911 678.761.9968 But that list is not complete. I made that list from doing *#61# on some friends' iPhones. So if you have ATT iPhone in the SF Bay Area and *#61# (then CALL) gives you a number that's not on that list, please post it here. I hope to compile a more complete list. (Of course, *#61# works nationwide on ATT, but I'm focused on the Bay Area. Also, none of those deposit numbers is in the Bay Area, but distance doesn't seem to matter for this at ATT.)
That's a GSM code and it's not limited to AT&T. Any GSM operator these codes will do the same thing. It's not at all limited to "iPhones" it's a GSM code that can be used in any GSM carrier. Some carriers such as T-Mobile have one number for their whole system. For t-Mobile it's 1-805-MESSAGE (1-805-637-7243.) For my T-Mobile Netherlands number it's +31 6 240634961. And by the way 253 is not bay area it's Washington state! Link for GSM codes
My iPhone with a Houston number calls +1(317)341-9000. That number appears to be a number in Indianapolis, Indiana.