This advice needs to be followed to the letter.
When we fly, there are a number of rules that have to be strictly followed. Among them is turning your mobile phone off or putting it in flight mode, and there is a good reason this rule exists.
What a specialist says:
It is strongly recommended to put your phone on flight mode when you fly. A pilot questioned on a blog specializing in aviation, Airline Updates, answered this question.
If you leave your phone how it normally is whilst flying, there can be interferences that can disturb radio communications between the aircraft and air traffic controllers. The high-pitched frequency that your phone will cause during these communications could actually be preventing an important message from coming through properly. This noise, as you well know, is similar to those that occur when you get close to a radio with your phone.
The consequences can be dramatic:
Therefore, it is easy to understand why it’s so unpleasant for people whose job this interference could potentially get in the way of. Keep in mind that your safety is in these people’s hands, so we advise that you follow what they say exactly! We’re certainly never going to disobey them.