AFAIK it is still illegal.
In an aircraft that does not have FM-immune systems, it is just plain stupid. What's more, to have your hands and brain occupied by making a phone call when your attention should be on flying is about as stupid as it's possible to get.
In-flight calls still on hold | CAA Newsroom | CAA
Mobile Phones | Information for Passengers | CAA
I also have serious doubts about the wisdom of listening to an MP3 player even when the radio is not receiving. I consider that it's important to be able to hear the note of the engine, any abnormal noises from airframe or engine, etc. For pax, of course, the situation is different. Keep the kiddies entertained, but don't be a kiddie yourself.