A. The mute button may be damaged due to the impact of the mute button when you are not paying attention. You can go to the repair center to replace the mute button to solve the problem. You can also set the virtual key to solve the problem.
Second, it could also be due to water ingress. If the iPhone into the water, do not turn on the use, do not restart, directly shut down. Then wrap it in paper towels and insert it into the rice tank, then turn it on after three days.
Three, is the reaction motherboard has a problem. You can replace the key row line to solve. Need to send the store for iPhone testing. If you can use it normally, this iPhone should have a hidden fault, do not know when the fault occurs. It's not easy to detect a hidden fault, so you can repair it when it happens.
If it's new or used, it has a warranty. Non-physical damage is covered by warranty. The official warranty is the most reliable and there is no risk of such dismantling as they do the original. The warranty can continue after the repair. At present, the general repair process is accepted by the iPhone store, return to the factory warranty, if directly in the store for you to dismantle it should be more careful. Because the iPhone 6S series above is a sealant, iPhone stores do not have the right to dismantle, you must return to the iPhone factory to dismantle the machine. In the store privately dismantled are very likely to be a fake iPhone store pretending to be official.