This addon is now available. Once added, it is called "Unlimited Family 4-Pack" on My T-Mobile. You have to call 611 to get it added. Make sure they remove the mobile to mobile addon if you already have that. He back dated the addon to the beginning of the current billing cycle.

You can add the option now even if you don't have UMA capable phones but utilize mobile to mobile. The cost is the same, and then the option will be there for you when you do get a UMA capable phone.