Earlier quoted context omitted.
As I said, I got mine for free. I don't know what they currently charge. It is supported by their Scam block service (which is free and can be enabled by shortcode): https://support.t-mobile.com/docs/DOC-4041#fourthheading
You probably got it for free because you were a T-Mobile ONE Plus customer (likely already paying a lot) [1]... otherwise it's an extra $10/month. And either way it wouldn't even solve the problem of getting too much spam at your current number... [1] https://www.t-mobile.com/news/digits-launch
Either way you asked for a VoIP number you could sign up to use places that don't allow them. That's what I originally answered.