I work at US Cellular so I actually see phones do this often. Most of the time we call into tech, & they say that after all logical troubleshooting, (replacing SIM, factory data reset, turn off& on again, resetting network settings) that there is just something internally wrong with device, as in loose internal antenna, and that there is nothing we can do other than replace the device. I however, used my resources & looked it up on Google to see what other people have done, tried taking out SIM & cleaning slot with canned air, returned SIM, & just gently hit phone on my hand a few times while it was off. Can clearly hear something is loose on inside, but when I turned device back on service was there & notifications were coming back on from past 3 days. Works for now, most likely temporary, but if I hadn't looked more into this, my customer would have had to replace phone for no reason! Extremely helpful, & happy customer for now. Device was also in fact an iPhone 6, which we've been seeing more issues with as well. Thanks much for being there even when Tech support (professionals) are not!!