If you're calling someone that has Skype, you could use Skype for the Wi-Fi calling, or another VOIP app. If you're calling a non-Skype number, you'll need minutes added to your account. If you've got Office 365, then you get 60 minutes per month free (not much there.) Just a thought.
I've never used anything for Wi-Fi calling nor had the need to. My understanding is that it helps when you don't have cell coverage. That's a T-Mobile thing for the most part, unless you live in one of those areas that NO carrier reaches, but you'd be less likely to have access to Wi-Fi there as well.
I know this post doesn't provide a T-Mobile solution, but lack of coverage and the need of Wi-Fi calling on T-Mobile is why I have stuck with a different carrier.