Voice message notification.

Hello Apple Support :)

I'm a tech support agent with a Canadian carrier, we are seeing more and more issues with voicemail notifications IE: MWI stuck / or no Message Waiting Indicator being received. I can assure you that the message is being delivered on our end, as I have access to the tool to check VM notifications. The issue is with the device.

Yesterday I removed my VVM to see if I would get sound when receiving a VM notification (to help a teammate TS).

1. I got a stuck MWI <---- I only managed to remove it by resetting all my settings. (I attempted to power cycle, soft reset, and soft reset while on AP mode without success)
2. Sound notification for VM is random, it worked 1/maybe 10 tries on 2 different devices.
3. Sound notif. works perfectly with VVM.

1. Any chance this can be looked into/ steps provided to manage the issue? I've seen it so far on iPhone X up to iPhone 13 and we probably get 10 trouble tickets/ day on the matter.
2. Is it expected behavior to not have a sound notification when receiving a voicemail notification? (sound and Haptics are set correctly, volume is on, Imessages notifications are working properly, etc.)