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.
In the Phone app , Visual Voicemail (available from select carriers) shows a list of your messages. You can choose which ones to play and delete without listening to all of them. A badge on the Voicemail icon indicates the number of unheard messages.

4. Save and delete messages. Voicemails will be deleted if the auto-save period is reached. The auto-save period can be from 14 to 30 days, depending on the type of voicemail.
To set up voicemail on your iPhone: 1. First, tap Phone. 2. From the menu that appears at the bottom, select Voicemail. 3. On the next screen, tap Set Up Voicemail. 4. Enter a password you want to use for your voicemail, then tap Done. 5. Select a custom voicemail greeting or the default one.