This tutorial is not mine!!! The credit for this goes to user Brad Herr. Don't thank me, thank him.
One prerequisite is that you have a micro SD card. If you don't have one, get one. Put it in your phone, and make sure you change your settings to install new apps on the SD. You'll also need the Preview For Developers app. If you already have it, delete it, and reinstall it on the SD card.
Next, you need to Developer Unlock your device. Someone put together a Lite version of the Windows Phone SDK which makes this easy.
Click here to download WP SDK Lite
There will be three tools installed with this SDK, but you'll only use two of them:
Developer Phone Registration Open Developer Phone Registration and follow the onscreen instructions. Essentially, you just need to connect your device to your computer and make sure your phone is unlocked and on the Home screen. After that, it's just a click. Easy.
Application Deployment We'll come back to this one in a bit.
Next, you need to download
Hack Tools.zip
This comes with two files: Custom_WPSystem.xap and Custom_PfD.xap
1.Deploy Custom_WPSystem.xap using the Application Deployment program from WP SDK Lite (follow on-screen instructions)
2.Take the Custom_PfD.xap file, save/place it in sd card using file explorer - DO NOT DEPLOY THIS ONE
3.Now, on your phone, open Custom_WPSystem.xap tool (should be in your app list if successfully deployed). At startup you can see “SD Card permissions” as red color. Now tap that, browse and select SDcard (just SDCard!), and then tap the check mark button at the bottom to apply permissions.
4.Now you can see “SD Card Permissions” as green color.
5.After changing SD Card permissions click on browse xap button (scroll down to bottom to find it).
6.Browse and select Custom_pfd.xap from SD card and click Apply XAP Button.
7.After successfully applying custom_pfd tool to preview for developers goto Preview for Developers apps.
8.Now your phone is hacked and you can edit registry of your Ativ SE.
Hacking the registry for MMS permissions
1.Open the Preview For Developers App
2.From the registry tree (should be the first pivot screen) tap HKLM\SOFTWARE\Microsoft\Messaging\PerSimSettings\Legacy\ADDR
3.In the text box type
http://mms.msg.eng.t-mobile.com/mms/wapenc
4.Click Write (Note: you won't get any confirmation, and nothing ont he screen will change; however, if you press the back button, and then tap ADDR again, you'll see the value changed to the url you just wrote - YAY!)
5.Tap the back button and navigate to WapPushTechnology. In the text box, type "0" and tap "Write"
6.Tap the back button and navigate to UAProf. Delete any text out of the text box and click "Write" (you want this value blank)
7.Tap the back button and navigate to IMsiAuthenticationToken. Delete any text out of the box and click "Write" (you want this value blank too)
8.Tap the back button and navigate to DefaultContentLocationUrl. In the textbox, type "http://mms.msg.eng.t-mobile.com/mms/wapenc"
9.Back into the registry tree, navigate to HKLM\Software\Microsoft\Cellular\MVSettings\IMSISp ecific\Default\CellUX\HideMMSAPN
10.In the text box type "0" and tap "Write"
11.Tap the back button and navigate to HideAPN. Type "0" in the text box and click "Write"
12.Back into the registry tree, navigate to HKLM\System\ControlSet001\Services\ICSSVC\Settings\EntitlementRequired
13.In the textbox, type "0" and tap "Write"
Congrats. Your MMS and internet sharing should now work on T-Mobile. You may need to restart your phone for everything to take effect.