WiFi calling on T-Mobile prepay

I just bought a phone capable of WiFi calling - Xperia XZ2 Compact. I have the $40 Simply Prepaid plan. I tried WiFi calling using the Digits app before I installed the SIM card and it worked fine. After installing the SIM, I can't get it to work.

Sony support guided me to the settings menu of the Phone app to see if WiFi calling appeared, and it does not. They said it doesn't appear when it is not provided by the carrier.

T-Mobile support said WiFi calling is not available with Prepay plans. However, their website says it does.

https://s3-us-west-2.amazonaws.com/.../2019/459221-Rebellion-Prepaid-PDF_110119.pdf

Any ideas how to get this working? Many thanks.

 

It's an unlocked phone. Not purchased from T-Mobile.

 

Oh yes. I spent some time on the phone and in online chat with T-Mobile reps. Their 1st excuse was that wi-fi calling was not available in prepaid plans. Then, a different rep said that it was not available on phones that were not purchased through T-Mobile. I was lucky to find a forum post from another user with the same problem, same phone, same network, and their problem was never resolved. So I lodged a complaint with T-Mobile, but without high expectations.

 

It seems you are correct. The question is, why?

If this is a policy intended to boost the sales of T-Mobile branded phones, you would think they would advertise it, instead of letting you find out after the fact.

If it is a fear that T-Mobile would be responsible for enabling WiFi on every kind of phone and that would be a burden to them technically... well I guess I can understand that. But they sell a few branded Xperia phones and I can't imagine that mine would be so very different.