What are you trying to do, the store locator or the coverage area? For the coverage area you would use the below URL to pull up the map:
http://www.t-mobile.com/coverage/default.aspx?zip=44301
Where 44301 is the zipcode from your application.
If I go to the page:
http://www.t-mobile.com/locator.aspx...2fDefault.aspx there is a text box there to provide my Zipcode but there isnt a zip code passed via the querystring when you enter or are redirected to the index. It looks like the page is setting a cookie or setting a variable in session to hold that zip code when the user posts the form.
To that end, what you need to do is write a bot that goes to that page, inputs the zip code, and posts the form back to T-Mobiles server THEN scrape the screen. What you are doing isn't causing the form to post or the zip code to be passed in, all it is doing is sending a zip code ot the page which the application is ignoring.
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I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
^^Thats my signature