Alright let me try to clear this problem as much as I can. From the parent carrier, we have a limitation or you can say a requirement that all numbers should have the Voicemail set up. So what we do for those customers who don’t want it, we disable it from our side but again the numbers keep syncing from the parent carrier’s network from time to time (roughly once a quarter). When the numbers get synced, voicemail automatically gets enabled. To some people, this is alright but sometimes it becomes inconvenience for the customers to call us again to disable it.
If you still remember the password of voicemail on iPhone, you can easily change the password of your voicemail on iPhone. When you forget the voicemail password, you should retrieve the information from the Email or call the carrier directly for a solution.

From your: Wireless phone: Press and hold 1 or the Voicemail key. AT&T Wireless Home Phone: Dial 1 from your connected home phone. Enter your voicemail password if prompted. In the Main Menu, press 4 for Personal Options. Press 3 to Change Greetings. Follow the voice prompts to record a personal greeting.
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