The mobile phone is unfortunately an essential tool of the personal trainer. A moderately successful PT will have 10-15 clients, and do up to 6 sessions a day, at least one of which will be rescheduled, cancelled, or have someone not show up to, and someone else wants that time slot, but hang isn't that one on holiday? And so on. You need to be able to call and text to arrange things, you certainly can't rely on the gym's front desk to pass on messages for you in a timely fashion. And you need the phone to check on new clients the day after their first workout, call them for their birthdays and generally stay connected.
But when the PT session starts, put the fucking phone away and don't answer it if anyone messages of calls. Check it after the person's session. The only exception I've made is when my wife was eight months pregnant, I told all my clients that if it rang I'd check if it was my wife or some close friend or relative, they laughed and understood.
"But what about using it as a stopwatch?"
Buy a stopwatch, or get a $15 digital watch that has a stopwatch on it.
No. Did I miss something? Have paper and pens stopped working?
If you as a trainer use your phone at all during the session, whether as a stopwatch, workout app or whatever, everyone watching will assume you're sending text messages to your buddies about your wild weekend. You just lost 10 potential clients, well done, perhaps for your next trick you could do a loud crotch grab on your client, unlike playing with your phone there may be a few who actually like that.
Your clients are paying $1-$3 a minute for your attention, that's what they want. Undivided. Fucking about with your mobile phone as you press the wrong part of the screen or enter the wrong number or try to find the exercise you had to substitute in because of the guy spending half an hour doing 4" range of motion 200kg leg presses will divide your attention. Don't do it.
Much the same goes for chatting to other trainers or gym buddies during someone's PT session. The obvious exception is when you introduce the client to someone and include them in the conversation. Don't talk over the head of someone bench pressing or anything like that. And don't knock out some chinups while the person is doing a plank. Seriously.
If you are not interested in what they're doing, why should they be? Focus, Grasshopper. Keep your attention on the person who is paying for it.