Really depend what type of job you do. If there is down times, then why not check your phone? Who cares...
When it comes to the Hobby, obviously just like everybody i wouldn't like my date to spend part of the session on her phone. It didn't happen often to me but i had one girl in particular i still remember who litterally made me wait while i was about to cum (me jerking myself while she wait for the cum) to answer a text message/notification. It wasn't the alarm or anything for times being up. I was pretty much on the bed, on my knee while she was on her back, im actively jerking myself and she just got up, im like "wtf...." in my head, it only lasted like 10 secs but it was the worst timing ever.
Otherwise i also hate when the sound is on. If you trying to have fun and have sex and every 30 sec you hear a "bling" it get very annoying super fast.
The only time i don't mind if she want to check her phone is when we just finished and im in the bathroom cleaning myself a bit, it usually take me 2 mins or so , so while waiting for me if she want to check if she had messages, then why not. But after that, i would feel weird if she would just check her cell while im laying near her... That would be awkward.
Anyway short version is, once your with the client, its like when the movie start in the threatre, cell should be on mute and not look at
But honestly a great majority of the SPs knows that and it SUPER RARELY happened that phone was an issue for me. Maybe 2 times total if my memory is good.