Hey, sillysilverlady! While any T-Mobile Branded device sold after 2006 is eligible to use the Name ID service, the Name ID app is an Android app. iPhone users can still take advantage of Name ID, but it does function a bit differently -- for example, iPhones will not display the phone number of the incoming caller. They'll usually display just the name (as if the person would be in your address book) but may also show Wireless Caller/unknown.
Right now this is a limitation with the way that the Name ID service integrates with Apple's existing proprietary features. At this point there aren't any timelines we can share for upcoming changes to this limitation, but we're happy to forward your feedback! Thanks for taking the time to post.