Frankly it sounds like she had an issue that no one was paying attention to the safety video rather than using a phone IMO.

I've had FAs on UA threaten to restart the safety video if they noticed people weren't watching it, and I've had FAs on both UA and DL request that all conversation stop whilst the video is playing.

I've noticed that UA and DL FAs tend to stand in the aisle/at the bulkhead while the safety video is playing, while AA FAs usually just camp out in the galley amongst themselves. I'm not sure if it is a specific policy at UA and DL, but I asked an UA flight attendant once (after they militantly marched up the aisle in unison right before the video to take up positions throughout the aircraft) and she told me it was policy to be in the aisle while the video was playing so passengers "would psychologically understand that their flight crews are not the same as the ones presented in the video" or something like that, which I thought was interesting.