Mobiletechworld has described in detail on how you can actually hack the phone to enable the dial up modem within your HTC WIndows Phone 7. First you will nede to jailberak it using ChevronWP7 unless you have a developer account. Next, you will nede to run DFRouter. Below is he instruction direct from the source.

As you have probably already noticed by now I usually only post about things that I personally think are worth talking about (that’s the beauty of having my own blog..) and this is why you won’t see me talk about every single homebrew Windows Phone 7 application popping up. Unless it’s something I think is really important like enabling tethering on HTC’s Windows Phone 7 devices. So if you have a developer unlocked device (you know..if you actually paid the $99) or if you just unlocked yours with the ChevronWP7 unlocker (which will definitely be rendered useless after the first OS Update) here is how to finally enable USB Tethering on an HTC Windows Phone 7 handsets:

1. Download XAP file here
2. Download HTC Sync for Android from HTC for USB Modem driver from HTC’s site (the whole package) or just the driver here
3. Deploy XAP file to your phone
4. Turn of your Data conection in your phone.
5. Run DFRouter and click on “HTC Modem”
6. If you only downloaded driver package just unzip to any folder, then Open Device Manager (recommended for Win7 64bit). If you installed the HTC Android package skip to step 10.
7. Find USB\VID_0BB4&PID_0EFF&REV_0000&MI_01 and force update driver with HtcUsbMdmV64 (inside unzipped folder)
8. Find USB\VID_0BB4&PID_0EFF&REV_0000&MI_00 and force update driver with HtcVComV64 (inside unzipped folder)
9. You will get two new device HTC USB Modem and HTC Diagnostic Interface (COM8)
10. Connect device to PC, open HTC USB Modem and select Advance Label
11. Add below command
(APN = Your carrier’s APN)
12. Create a new connection with HTC USB Modem, diag number is *99#
13.Just connect without user name and password.