inputting phone no. in settings

I have an old iphone 4 that I've just set up on T-Mobile.  It all works well, I've managed all the settings and have it set up so it makes and receives calls.  However when I go, in settings, to input the new number in 'my number' it responds with 'can't save this number.'  Without that setting texting is not possible on the phone.  How can I fix this?