I actually had a conversation with an FA on one flight. I was in 1A on a little Canadair, and her jumpseat faced me, so we talked. Summary:
Cellphones cause absolutely no problem in and of themselves.
FA's text the whole flight anyway.
The requirement is to maximize their chance of you paying attention during the safety briefing and in case SHTF.
That said, I've had more than one co-worker tell me about riding through an "auto-landing" in zero/zero conditions, and the FA's came through to verify that all devices were physically powered off, because THAT situation can be affected by cell devices.