Answer: The most likely cause is the obstacles.
Step-by-step explanation:
Since, the user is experiencing problems intermittently then, it is sure that she is loosing her connectivity again and again. Connection lost can happen because of many issues but here in the given situation it is because of the hall the elevator.
It is difficult for her to device to receive the signal and the strength of signal is lost while reaching the device. As, signal travels from one point to other, it tends to lost the strength thus resulting in poor connection.