Please turn off your handphones. Our flight will be departing soon

I am wondering how serious is the threat of keeping our handphones turned on, or even taking calls whilst on the airplane. I mean, if it really interferes with the aircraft system, then shouldn't more stringent procedures be in place to ensure that ALL handphones are turned off, instead of the obligatory safety briefing given to passengers prior to take-off? Surely, they could be passengers on board who are not aware of the need to switch off due to various reasons, e.g. they may not have heard the request to switch off, or they may simply have not understood the request due to language barriers! What do u think?