If hes out of the room, sending a quick text is fine but I wouldnt recommend being on the phone when hes out of the room let alone when he comes back into the room. Men dont want to feel like your running a conveyor belt service and that would be the impression you would give them if your on the phone. Even though they are fully aware that we fuck other men, they want to feel like you are exclusive to them
You might want to consider downloading an auto text app so that when you are on a booking, anyone who calls or texts you gets an auto reply. Its up to you what message they get back but it could be someone along the lines of "thanks for getting in touch, I will get back to you as soon as possible when Im available".