There's a thread over on xda-developers with a growing number of users who are experiencing this issue. I've been in contact with T Mobile, they've swapped my sim card but the issue persists.
The Huawei Mate 20 pro is an unlocked international phone. The first issue I experienced was with Visual Voicemail. The post is located here .
If you read the post you'll find that another user found a solution to this problem which seems to fix it, for the most part. I'm still experiencing issues with losing cellular data for no apparent reason from time to time. The issue improved to the point that it's tolerable, but it's still there.
The current problem:
Some calls are routed directly to voicemail for no reason. It's hit and miss and exceptionally troubling. Every user that has reported this issue is using T Mobile either directly , or indirectly (through a reseller). I've tried everything from resetting the phone and using it without Visual Voicemail installed to changing the APN to swapping sim cards, nothing has worked.