[T-Mobile HTC One (M7)] weslymoore:
What worked for me

This is what worked for me. This informing following info was found in a Tmobile forum. The 1st step likely isn't necessary but was done during my stumbling to a solution.

1. **004*1(Google voice number) #

Again that part may not be necessary.

2. **61*1( Google voice number) #

This command forward calls if no reply

3. **62*1( Google voice number) #

This command forwards calls if not reachable

4. **67*1(Google voice number) #

This command forwards calls when busy.

Note: to check status of forwarding use the following commands .

*#61#

*#62#

*#67#

To cancel forwarding use the following commands .

##61#

##62#

##67#

The following command changes forwarding delay.

Also,it is untested by me so I'm not certain if the forwarding number should be changed to the Google voice number or left as is try at your own risk.

**61*18056377249**seconds#

(For example, 5, 10, 15, etc.)[/QUOTE]

Thanks to all who posted on this topic. Since I did some fairly extensive testing along the way, I thought I'd share what worked for me (HTC One on T-Mobile, using GV just for voicemail handling).

Like CJohnson1073, I found that in the end ALL I needed to do was step 1 above. (Note that this is what GV tells you to do when you click to active GV on your phone via the web interface. Like others, I was not able to activate successfully via the app.) Steps 2-4 do have individual effects, but for some reason they seemed to introduce some inconsistency in the way calls were handled, especially differences between when using wifi calling and when not. As an example, at times some calls went to a busy signal rather than GV voicemail. This didn't make sense to me, but since I ultimately found a setup that's workable (enough) for me, I called it a day rather than experimenting further.

Following step 1 ONLY (for me) forwards calls to GV for voicemail handling, rather than to the carrier's voicemail. This works both on the cellular network and when wifi calling is active (an absolute requirement for me). The only drawback I see is that callers go through two ring cycles, one for the carrier and another for GV. So far I've found only a partial solution for this, not acceptable to me. From what I've found so far, the GV delay is built in and fixed at 25 seconds, so there's no changing that. You can use the **61*... instruction above to change the carrier's delay. (From what I've read elsewhere, this works for T-Mobile and AT&T, but not for Verizon. You can also check the current setting with instruction *#61# .) As with the other instructions, wifi calling must be OFF so the instructions can be executed. The telephone number (805... above) DOES need to be your GV number or carrier's voicemail number. This does work, technically, but the drawback is that reducing the delay in seconds (to reduce the overall wait for callers going to voicemail) necessarily reduces the number of times the call rings on your device before going to GV, and thus reduces the amount of time you have to answer the call. This didn't work for me, but it may work for you. Right now I am living with callers getting eight rings before voicemail. Hopefully my friends are patient!

I did find elsewhere a suggestion to change your GV settings for the phone to send calls directly to voicemail, to reduce/eliminate the "extra" rings. This did not seem like it would apply to the way I'm using GV, but I tried it anyway. It didn't make any difference in the number of rings the caller hears.

The one thing I have noticed is that missed call notifications only come from GV if the call gets routed/forwarded to GV. This makes perfect sense when you think about it, but it's worth keeping in mind if you find yourself wondering (as I did) why some calls show up as "missed" in your phone's call history, but not in GV's. Most likely this means you declined the call or the caller hung up before the transfer to GV.

I might have been happy enough with T-Mobile's visual voice mail - hey, it's a pretty well integrated solution, not unattractive, and for what it does it works - but I'm used to the convenience of GV text transcripts (not to mention being able to route them to email), and I'm not willing to pay T-Mobile another $3.99/month for something GV provides for free.

As always, even when tech is involved, your mileage may vary. :D