Intermittent Incoming Calls Going Straight to Voicemail with No Ring

What KF6OLQ says is accurate. Been dealing with this for years. And I use Google Voice as my voicemail. "I believe" the problem is with the Supplementary Call Services, the Call Forwarding rules: Call Forward Busy, Call Forward Unanswered and Call Forward unreachable, which all have a default 15 second timer. Basically the system waits 15 seconds before initializing said action. The question is when does this timer begin. By extending the timer out, it's definitely helped, at least in my case. I have mine extended out to 25 seconds. T-Mobile has self service short codes you can use to change this timer.

https://www.t-mobile.com/support/plans-features/self-service-short-codes

**61*15555555555**25#    Call Forward No Reply

**62*15555555555**25#    Call Forward Unreachable

**67*15555555555**25#    Call Forward Busy

Even though the short codes site only shows 61 being able to extend the timer, I tried it with the other two code 62 and 67 and it all took. **61*, **62* and **67* is the condition. 15555555555 is the number to your voicemail. **25# is the delay. From the site, shows you can use 5, 10, 15, 20, 25, 30 seconds.

Go to the phone dialer, enter the full code (make sure you enter your voicemail number) and dial (green phone icon in my case). You should see a quick pop up towards the bottom saying MMI Code Started. Then get another pop saying Registration was successful. If you get a failure, try it again.

The only downside to this is the caller on the other end is going to have to wait longer before they're sent to your voicemail. If this works for you, consider this a band-aid and not a solution.