A phone is a radio broadcast device. If you're sending something unencrypted from it, anyone nearby can listen in to what it's sending. Of course, it's all compressed and sent with different protocols depending on what app you're using, so it's not trivial to read messages from everyone to everyone all the time, but if someone is determined it's quite doable. SMS messages in particular are famous for having that happen to them, but it can happen with any unencrypted message.
A phone is a radio broadcast device. If you're sending something unencrypted from it, anyone nearby can listen in to what it's sending. Of course, it's all compressed and sent with different protocols depending on what app you're using, so it's not trivial to read messages from everyone to everyone all the time, but if someone is determined it's quite doable. SMS messages in particular are famous for having that happen to them, but it can happen with any unencrypted message.