To resolve the "Inbox is full" issue, verify that you are deleting messages saved on the AT&T voicemail server instead of those saved on your iPhone. You can do this using one of the following steps:

• Listen to and delete the recent voicemail messages on your iPhone.
• Call your AT&T voicemail by pressing and holding 1 or by dialing your own phone number, then listen to and delete the messages directly from the AT&T voicemail server.