It seems 11-digit authorized phone numbers that start with 1 don't work. (I tried 11-digit numbers that started with other numbers, as well as 12-digit numbers that started with 1, and those both worked.) Should a line be added to 183 to this effect? Something like "It is not necessary to include the 1 before your phone number; authorized phone numbers that are 11 digits and start with 1 will not work."
Maybe something should also be added to the Voice Post Settings page, since that's where people enter in their phone numbers.