At this point, we can’t be exactly sure why your iPhone won’t play voicemails. The voicemail you play in the Phone app on your iPhone is called Visual Voicemail, which downloads your voicemails from your carrier in the form of little audio files, similar to the music files you listen to inside the Music app.
T-Mobile: T-Mobile has a similar story to tell as Verizon. Instead of allowing you to set up a specific basic voicemail, it instead directs you to Apple to set up Visual Voicemail. I dont see anything that you have written when I try to set up voicemail on my new Iphone11. I tap on voicemail and the only option is 'Call voicemail' … then it doesn't accept my mobile number.

By using the reset network settings, an effectiveoption to solve network related issues, you can fix allthese problems by simply resetting the networksettings of your iPhone as it will clear all the networksettings, current cellular network settings, saved Wi-Finetwork settings, Wi-Fi passwords, and VPNsettings How do you reset your voicemail greeting? From the Home screen, tap the Phone icon . Tap Voicemail then tap Greeting (upper-left). Greeting islocated in the top-left corner of screen. Tap Custom to record a greeting. Enabled when a checkmark ispresent. Tap Record to begin recording the custom greeting message. Tap Stop to end recording then tap Save. How do you change voicemail settings on iPhone? Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now. Create a voicemail password, then tap Done. Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can recorda new greeting. Tap Done to save your greeting.
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.