Every time I need to set up Google Voice on my HTC One with T-Mobile service, I get it a little wrong. I think I finally have it, though. The process turns out to be very fragile.
When I install Google Voice, I get a failure message trying to set up T-Mobile call forwarding. This is normal and expected.
I first need to turn off the wireless on my phone. (Thanks ebaychuck!) This really does seem to matter.
Going to the Google Voice page and into the settings, there's an option to disconnect my phone from Google Voice. I select it and follow the instructions, which involve sending a short code through my phone.
I then push the button again to reconnect my phone to Google Voice. Again, I enter a short code. The "short code" is pretty long this time, since it includes my Google Voice number. Note that it is prefixed with "1". This may matter.
I always want to control the number of seconds before my phone forwards.
I go to T-Mobile's Call Forwarding Short Codes page.
Down at the bottom there's a code to set the timing. Note that the phone number embedded in that code is an example phone number. I need to use my Google Voice number (prefixed with "1") there instead.
Having followed all these steps, Google Voice seems to do the right thing with the phone.
Bleah. Very, very fragile. (B)