So, I've seen a few posts on this out there but nothing this simple and all in one place. So here are the simple instructions for manually installing the TMobile Service Books to your 9800 Bold. You'll need Desktop Manager 6 installed (Older versions may work but the instructions below are for DM6). This procedure for the BIS browser may also be necessary for a while with the 9700 if you have loaded OS6.
BIS Connection and BIS Browser:
**Update 1/21/2011: It appears that this step is no longer necessary as TMobile now has the service books for OS 6. They should push to the device automatically when registering the handheld device.
1. Download and unzip the TMobile Service Books below. Be sure and remember where you unzipped the folder to as you will need it later.
2. Next, make sure your TMobile sim card has been inserted in the Torch. Then go to Options >Device >Advanced System Settings >TCP/IP
Set as follows:
APN: epc.tmobile.com
Username: < Leave Blank >
Password < Leave Blank >
3. Go to Options >Device >Advanced System Settings >Service Book.
Do the following:
Type alt + s-b-e-b. You will not see anything as you type this but when you're done you should see a message that says "Legacy SB Restore Enabled." Click OK.
4. Plug in the Torch to your PC and open Desktop Manager and go to Device >Restore
5. Under File Name you should see 9800 TMobile Service Books. If you don't see that click on "Change Folder" and browse to where you downloaded the service books from the link above.
6. Toward the middle of the Restore Window click the radio button next to "Select device data and settings" and below that make sure that "Data Type" and "Service Books" boxes are checked.
7. Click the "Restore" button and you will be prompted that "Your current data will be replaced." Click "Yes."
8. Once the restore is finished you can disconnect the device.
9. On the device go to the "Setup" icon. Select "Email Accounts." Select "Internet Mail Account." Hit the menu key and select "Service Books." Click "Send Service Books." This will resend your email service books.
That's it. You should now have the Internet Browser and your apps should work normally.
Now for UMA!
**WARNING*** You can severely mess up your device by changing settings in these screens. Be sure you are comfortable with the procedure before attempting.
1. Download the certificates for TMobile from here: (use the links in the second post)
http://www.port3101.org/port-3101-be...tificates.html
2. Now you'll need to unlock your engineering screen. To do this click alt + rt shift + H on your keyboard to get to the "Help Me" screen and fill in the info from that screen into the key generator at the site below. Don't back out of this Help Me screen until you enter the code and get the new menus. If you back out you'll get different uptime and you'll have to re-enter the info and get a new code:
You'll need the PIN, App Version including the (xxx), and the uptime without the "secs". Enter that info and choose the amount of time you'd like to keep the screen available.
Zibri's Blog: Hidden things are usually the best 
Once you get the code, simply type it on that same screen. Use alt when typing numbers and don't use caps with letters. You won't see anything being typed as you enter it. Once the code is accepted you'll see a new menu list. If not, go back and make sure you are entering all the info correct and try again.
3. Once the new menu appears, click on "Mobile Network Engineering Screens"
4. Click on "Utilities"
5. Click on "Session Manager"
6. Press the menu key (left of trackpad) and select "UMA Options"
7. Press the menu key and select "New"
8. Enter the following:
Display Name: T-Mobile
Operator Name: T-Mobile
9. UNCHECK "Use default UNC address"
10. At "Provisioning UNC address:" enter: punc.t-mobileuncs.com
Leave Provisioning UNC port: 14001 as is.
11. UNCHECK "Use default SEGW address"
12. At "Provisioning SEGW address:" enter: psgw.t-mobilesgws.com
13. Under "SEGW certificate" choose the TMobile Engineering certificate downloaded in the Step 1.
14. Under UMA protocol version: use "UMA v. 1.0.3"
15. Save your settings.
Reboot the device. UMA should now be enabled!
New: 12/03/2010 Added Tether Support with DM6
So, now you have BIS working and UMA working but why is it that you can't connect to DM6 and get the automatic Tether options? ATT blocks tethering with Desktop Manager. However, with a simple edit of an xml file we can add that feature back so you can Tether your AT&T 9800 unlocked on TMobile. I can't take the credit for this, however. The credit goes to
jeoleapus on RIM Forums from his post
here.
1. Install DM6. You can get it here:
Desktop Manager
2. Make sure Desktop Manager is closed.
3. Search your computer, including hidden files and folders for the following file:
ip_modem_configuration.xml If you don't know how to show hidden files see the links below:
WinXP
http://www.microsoft.com/windowsxp/u...ddenfiles.mspx
Vista/7
http://windows.microsoft.com/en-US/w...w-hidden-files
In Windows XP you can find the file located in C:\Documents and Settings\All Users\Application Data\Research In Motion\BlackBerry
4. Right click on the xml file and select "Open With" and select "Word Pad." You can use any text editor you wish but Word Pad seems to open it in a decent format.
5. Look for the following:
<carrier uid="102" id="AT&T" enabled="
false">
<vendorid>102</vendorid>
<supportednetwork>GSM</supportednetwork>
<apn>isp.cingular</apn>
</carrier>
Change it to the following:
<carrier uid="102" id="
T-Mobile US" enabled="
true">
<vendorid>102</vendorid>
<supportednetwork>GSM</supportednetwork>
<apn>
epc.tmobile.com</apn>
</carrier>
Open DM6 and connect with the USB cable.
You should now see "Mobile Internet" at the bottom left of the DM main screen. Click on that and just use the default Setup info on the screen. It should list "T_mobile US" and should work normally.
Again, a big thanks to joeleapus over at RIM forums for this fix.