If you call your regular voicemail number by pressing and holding the #1 on the telephone keypad, you have the option to listen to and save voicemail messages there. The default is 30 days, anything longer requires you to select another number. Not sure exactly which on it is. This should preserve the voicemail(s) for you. They are stored on the AT&T server, and once they are deleted there, they will be deleted on the phone.