07953 966150 is the number given on
T-Mobile's contact page and when I call it from my BT line I get a message telling me I have dialled an incorrect number.
However, if you are calling from a landline, then you can access the normal customer services menu that you get when calling 0845 412 5000 by ringing the number for sales, 0808 121 9999, and selecting option 2.
I have amended the T-Mobile entries to explain this.
More information on T-Mobile's locations, including switchboard numbers for its offices here:
http://www.t-mobile.co.uk/services/careers/about-us/our-locations/