If you read/hear about something you shouldn’t have whats the ruling/classification of discussing it with others, such as reading a text message that was sent to someone elses phone, and if one accidentally does so what should they do?

It is not allowed to read someone else’s messages or listen in on them, as this is the spying Allah forbade. What reaches you without intending it is not spying, but keep it hidden and do not talk about it, because covering a hidden fault is obligatory and exposing it is not allowed unless in certain situations, like when you are asked by a sister's wali about him because he proposed, so here you have to tell them.