This seems to be a common problem. I tried all the basic software fixes like restarting the phone, resetting the network settings and even completely erasing the phone. Blowing on the SIM card and slot, even dropping the phone which seemed silly but nothing was working. Then I came across one post that turned out to fix my problem. Hold down the home button and the power button at the same time until the phone shuts off. Continue holding both buttons until the phone restarts and shuts off once more. Then let off the buttons. Wait long enough to be sure the iPhone shut off completely and then just turn back on. This fixed my iPhone 6 Plus for good. Good luck.