How to unlock an iPhone that's been reported as lost?

I found my old iPhone the other day but because I reported it as lost I can't unlock it. I called Apple to ask if they can unlock it if I provide them proof of purchase but they said there's nothing they can do. There's stuff on that phone that I need!

All you need to do is enter your phone's passcode on the lock screen. Failing that, sign into iCloud with your Apple ID and turn off lost mode from there, or erase the phone and remove it from your device list to start over from scratch.

I'm still trying to figure out

How people Lose Phones (and remotes).

It just Doesn't Seem Possible to LOSE a $@#!&* - []@/\/\><&#!$@ PHONE!

"I found my old iPhone" -Correction. You STOLE the iPhone.

If it's your phone, you would have the password to it lost or not. If you do not have the password the phone is worthless.

You should check your carrier about that.

Though, in the case of the carrier, reporting it lost doesn't mean they could lock it, in the way you are seeming to describe it being locked anyway. That could only be done if it was locked via Find My iPhone, but in that case you should know the necessary credentials.