i maybe wrong, but i dont think that can work. From what i understand, the data/cell conncetion will work with your signal, and if your using the signal for calling, data will start to download as well.
I'm suprised your having issues with the sounds quality of wifi calling. It's seems flawless to me. Have you tried a hard reset to see if your having the same issues with wifi calling. If your still having issues, maybe you can exchange your phone for a new one since it's fairly new and see if the issues presist
I could not live w/o it as the signal is very low for tmobile in my home.
Hope ya find an answer to your issue.
Yes you can connect the myTouch 4G Slide to your wi-fi network for the data connection and force all calls through T-mobile's cell network.
To do this you need to change the Wi-Fi Calling feature to "Cellular Preferred" or off entirely. See the instructions below.
When you're in a place with limited cellular coverage but with Wi-Fi® connection, use Wi-Fi Calling to make and receive calls over Wi-Fi using your available rate plan minutes.
Turning on Wi-Fi Calling
Make sure that myTouch 4G Slide is connected to a Wi-Fi network before you turn on Wi-Fi Calling.
1. On the Home screen, tap > Wi-Fi Calling.
2. Read the important notice about emergency calling, and then tap Dismiss.
3. Tap the Wi-Fi Calling switch. myTouch 4G Slide then connects to the T-Mobile network and shows Active on the screen when connected. You can then go to the Phone dialer, Call log, or Contacts app to make calls.
Changing Wi-Fi Calling settings
1. On the Wi-Fi Calling screen, press and tap Settings > Connection preferences.
2. Select a connection option, and then tap OK.
When you make calls, the Wi-Fi network is preferred over the cellular network. You must stay in the Wi-Fi range when calling.
When you make calls, the cellular network is preferred over the Wi-Fi network.
myTouch 4G Slide will always use the Wi-Fi network when you make a call. You must stay in the Wi-Fi range when calling.
Turning off Wi-Fi Calling
1. Press and hold the status bar, and then slide your finger downward to open the Notifications panel.
2. Tap Wi-Fi Calling Ready.
3. Tap the switch to turn off Wi-Fi Calling.
Source: Page 43 of the user manual; http://cache.mytouch.t-mobile.com/utils/downloads.ashx?file=user_manuals/mytouch-4g-slide-user-manual.pdf