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...