Why can't I erase all the messages on my iPhone?

IPhone supports deleting conversations under the same contact. You can manage SMS conversations in two ways:

1. Clear conversation record: delete all information in the current conversation, but keep the contacts in the short message list.

2. Delete conversations: Delete conversations and contacts in the SMS list.

When a conversation is cleared or deleted, all messages related to the conversation will be cleared or deleted. You cannot clear or delete individual messages in a conversation.