How to I get a text back I accidentally marked as junk on my iPhone?

Report spam or junk messages

In the list of messages, press and hold the spam message, then tap Report Junk. Or, if you've opened the message and the person isn't saved in your contacts, scroll to the bottom of the message, tap Report Junk, then tap Delete and Report Junk.

Block, filter, and report messages on iPhone - Apple Support