The 901 phone by O2 can store up to 40 minutes of voice messages. Upon reaching the phone's storage limit, however, the phone's user will receive a notification. At that point, they can either manually delete all the voicemail messages. Unfortunately, no automatic reset exists for this type of phone. It is also possible to reset the phone's voicemail greeting through this process.
Press 9-0-1 using the phone's keypad to access the phone's voicemail.
Press the star (*) button once you hear the greeting.
Enter the phone's voicemail pin using the keypad. This is the same number assigned to the phone's voicemail system upon activation.
Press option number three to change mailbox settings.
Select "Change Greeting" to reset the greeting that's already stored on the phone.
Follow the prompts to re-record another greeting, or change to a standard pre-recorded greeting.
Dial 9-0-1 using the phone's keypad to access the phone's voicemail.
Enter the phone's security pin.
Manually delete voicemail messages by pressing three on the keypad. It's necessary to repeat this step for every voicemail message to empty the inbox.