It can be a sign that he hasn't came recently, but it isn't for sure, some people just orgasm faster than others and there's also a condition called premature ejaculation (PE)
You have no way of knowing he isn't watching porn while on the phone too. Men typically come faster masturbating to begin with. And as to the question there is absolutely no way of knowing. Men can cheat and have sex with a second woman in the same day and come in 5 minutes, like, you definitely have more information and intuition on this situation than any of us.
There's no way of knowing tbh