Text messages are stored forever (or until storage arrays are full) on the SMS gateways for the provider. They are stored in plain text and are backed up if the admins are doing their job. You can grep on MSISDN or text strings.

The only time you need access to the phone is for large images, videos, etc. Or if they are using a web service to chat, then you have to subpoena that provider for their chat logs, which almost everyone keeps.