Our phones GSM bands interfere with unsheilded radios. The effect is most prominent when you’re getting a call and you’re close to a hifi system.

Often you can tell that your phone is about to ring before the phone even begins to vibrate or chime.

https://www.quora.com/Why-do-speakers-make-noise-when-mobile...

There is a high likelihood that this isn’t a problem now and hasn’t been a problem for some time; but it might be wise to err on the side of caution if you want your pilot not to have to repeat or request repeats of radio information.

And once you’re in the air your phone will frenetically try to connect to a cell tower, absolutely murdering battery life. So you should turn it off after take off anyway just for your own battery sake.