Just that really!
Me and dp went to the spa near to us today, quite a naice place, had a massage each and a couple of hours in the general spa facilities.
Over the course of the afternoon we saw at least 3 other guests in the spa area on their mobile phones. Most were not on the phone (ie talking), but just (i assume) texting, browsing online, playing games etc. One girl was even in the sauna with her phone, and we also saw a bloke in there with full headphones! 
But even WORSE than that hoicks judgy pants was the guy who was walking round facetiming someone! I could hear him from inside the sauna: "that's the pool, here are the showers" etc etc.
So why would you pay to go to a spa and just sit there on your phone?
and worse, why would you go and facetime someone else, loudly, and disturb other guests?! Just relax and enjoy the quiet 
In fairness I would understand if you were awaiting an urgent message/call, though me and dp discussed this and struggled to think of a situation that was urgent enough you'd need your phone, but not want to cancel a spa day...