I have run around with Apple and Verizon about this issue for some time. Let me list the problems I'm having, all related to VoLTE and the XS Max.
- Calls drop after 2 seconds into phone call, with repeated phone calls all dropping with phone alerting "Call Failed"
- Dialing a number and hearing a recording from Verizon saying "Welcome to Verizon Wireless, the called party is not available"
- Dialing a number and after pressing send seeing a clickable dialogue on the phone saying "Last line no longer available. Do you want to call using your remaining line?"
- Some inbound calls going strait to voicemail.
Again, all this can be fixed by turning VoLTE off. However this is not a suitable solution as I loose internet connection while on phone calls.
Also note that this is happening to my wife as well and we have both replaced our phones with new ones from Apple. As well as reset our phones and set them up as a new phone. (My personal favorite 5 hours of wasted time solution to every problem from Apple)
I assume we will see a solution to this in a future update with no acknowledgment from Apple, who knows, maybe they will surprise us.