We're sorry to hear about that frustrating problem. Most often, this is caused by a lack of signal or a malfunctioning SIM card. We can activate a new SIM card for you at a T-Mobile retail store should that be the case.
Also, in case call forwarding was activated in error, you can dial # # 0 0 4 # (POUND POUND ZERO ZERO FOUR POUND) from your phone to cancel any automatic call forwarding.
Thanks for posting and have a great day.
I have exactly the problem. I have two iPhone 5c and one iPhone 5s that show the same behavior. T-Mobile told me to call apple support. I went through all of their troubleshooting guides and wiped and restored the phones and nothing works. The first call from a given number within about 1/2 an hour always goes to voice-mail. If the caller tries again within 1/2 an hour it will ring, but if they wait longer than 1/2 hour they'll get sent to voice-mail again. This has been going on since I switched from AT&T. I'm going to have to leave T-Mobile and go to another carrier. The phones are borderline useless like this.