Is it a red flag if they spend the whole date on their phone

Phone use during a date is annoying, but it does not automatically mean disinterest. Someone can be anxious and scroll to calm their nerves. They could have work stress they are managing poorly. They might have a habit they have not thought about. They might be someone who checks their phone constantly around everyone.

The real signal is whether this is a one-time thing or a pattern. If they are on their phone the whole time on date two and date three, that is different. One date tells you something happened that day. Multiple dates tells you who they are. Wait for more data before you decide this is a dealbreaker.

Do not let this go unmentioned. Staying silent tells them it was acceptable. But do not deliver it like an accusation either, which makes them defensive and less likely to explain or change.

The move is to describe what you observed in a plain, factual way, then ask a real question. Say something like 'I noticed you were on your phone a lot during the date' and ask what was going on. This gives them room to explain without you having to guess. It also shows you are direct enough to address things that matter to you, which is attractive.

Do not reply to their response in the same message or get into a back and forth about whether they were really on their phone. Let them answer. Listen to what they say and how they say it.

A good answer includes some accountability. They might not be totally aware, but they notice you felt disconnected and they care. A bad answer is deflection or dismissal. If they say 'I was not on it that much' or 'you are being too sensitive', that tells you something real about how they handle feedback. After you have their answer, you can decide if you want another date. If they apologized and seemed to mean it, give them another shot. If they got defensive, you have your answer.