Edge not working on T-Mobile network for unlocked iPhone?

Question: I have 3G iphone v.2.1. It's unlocked to use t-mobile. I was using T-zones or called T-mobileWeb ($5.99). Everything worked well for many days. But all of sudden, EDGE network does not work anymore for me....The 'E' sign doesn't show up anymore from today.. I dont know why.. it just stopped working from last night. thank you!

Answer: This happened to me as well. I figured out a way to enable the 'E' again:

1. I assume you have followed my previous post about "turn off 3G, turn on Data Roaming, set APN to epc.tmobile.com". And now the 'E' disappeared for no reason. Follow the steps below:
2. Turn off "Data Roaming" in the General->Network setting
3. Power off iPhone by holding the power button for 5 seconds, the red slide bar will show up, slide the red button to power off
4. Power on iPhone
5. Wait until the T-Mobile sign appear and signal bars are up.
6. Now go to General->Network setting to turn ON "Data Roaming"
7. 'E' should appear again.

After step 7, if the 'E' still doesn't show up, add an extra step 1.5 (between step 1 and step 2) "step 1.5, take the SIM card out of iPhone and put it back in again." Then follow step 2 to 7. That should re-enable 'E'