how can i get the phone to show old messages? on iMessage's?

Are these messages you deleted? Or just very old messages? If you mean very old messages, first, go to Settings>Messages>Message History>Keep Messages. Does it say "Forever"? If so, the messages may be there but very difficult to access. The easiest way to access very old messages would be using software such as iMazing (Mac) or TouchCopy (Mac or Windows). Both will allow you to archive your messages to your computer as a pdf that will be much easier to navigate. The applications aren't free but, they can be invaluable.

If you mean you deleted the messages, how long ago? If you have iOS 16 on your phone and the messages were deleted less than 30 days ago, you may be able to recover them:

Recovering deleted texts and iMessages - Apple Community

Delete a message or conversation

When you delete a message or conversation, you can't get it back. So make sure that you save important information from your messages.