I used to work for AT&T.
If the person is spoofing their caller ID, then you're pretty much screwed. The telephone system was not built with authentication in mind. That combined with calls hopping across networks, it's pretty hopeless for you to be able to identify callers.
Assuming the person is not spoofing their caller ID, and is just using a telco service like *67, then your only recourse is T-Mobile. You cannot unmask this data on the phone because the data is not there in your phone to unmask. However, T-Mobile definitely knows who it is. The data is there for them and if the call goes to an authorized entity like 9-1-1, the number won't be private. So if T-Mobile customer support is not helping you, you need to escalate to someone with better capability. I suggest threatening to switch providers and the customer retention specialist may work on your case. Phone companies hate churn (customers switching back and forth)--it's one of the metrics our business is measured on and it impacts our stock value.
Regardless of what's going on with T-Mobile, I would attack this on multiple fronts. Get a lawyer and have them write a "cease and desist letter". It should mention that you have her voice recorded trying to incite you to kill yourself, and that this is evidence of criminal reckless behavior. The letter should insist on a cease and desist to the calls, and indicate that if this does not occur, there is possibility of a civil suit and the possible penalties for criminal reckless behavior. A lawyer letter is a very common thing that lawyers are used to writing, and it should cost you a flat fee of about $200-$300. It doesn't matter if you do not intend to follow through or not because the point is to scare her straight with the letter.
There is actually a technical solution to what you are asking for. I didn't start with it up front because it's not really meant for cellphones and the learning curve can be pretty big--it's not a simple app. Most businesses with lots of calls have a "PBX", which is a local private telephone network. The PBX provides a massive amount of custom control including automatic routing, interactive voice response (phone menu), more numbers than there are lines (via extensions), and a lot more. But to your point you could configure private calls to not ring and not go to voicemail, but instead just play a recording that your business doesn't accept calls from private numbers. Legitimate callers who have a private number will know to call back with *82, thus "unhiding" their number. Again, this isn't really meant for cellphones, but now that cellphones are powerful computers there is in fact software you can install that emulates a PBX virtually. I have no experience with this, so I cannot recommend one, but I suggest researching PBX software for Android.