It's anticipated that it will be available starting tomorrow (May 11) for T-Mo subscribes with Even More, Even More Plus and 4G Do More plans. Customers with these plans will be notified via SMS for the new feature and will be redirected to T-Mobile's site. If the need arises (or if you feel too lazy to add it yourself), you may also call the Customer Care service.
Of course, you must own a phone that supports Wi-Fi calling (shocker!), but these are all the requirements you need to cover to get that nice feature. As of now, T-Mo subscribers are using their in-plan minutes when making calls over Wi-Fi, but it seems this is very soon to be a thing of the past.
source: T-Mo News