Depending on if you came from Android to iPhone I would think that would have been a voice mail handling issue with apple. However if you were coming from iPhone upgrading to an iPhone I would still go to Apple. If you came on as a new customer with this phone at T-Mobile then I would be inclined to get together with both T-Mobile and apple.
As far as the spam and scam calls go do you have that protection enabled on your account for free? Also there are a few apps out there that work fairly well to block these kinds of calls and T-Mobile has already begun implementing spoof protection.
I'm using an Android on T-Mobile with scam block and spam ID and I also use Hiya which works in real time on Android but due to limitations with apples OS most apps can only identify the call after you hang up.
I'm also fortunate enough to use a phone that uses Google's phone app which IDs businesses and screens scam calls. Obviously I have much more protection than the normal user however using T-Mobiles free services and a third party app which works with apples call kit you should find more satisfaction.
What specifically is going on with your voicemail? Are people hearing invalid number when they go to your voicemail?
What kind of speeds are you getting on your phone and how many bars of signal does your phone show that you have? Do you have an LTE next to the bars?