How do I unlock iPhone 15 after forgetting passcode?

You’ll likely need to connect your iPhone to a computer and erase it, then restore and set a new passcode. 

Start here: If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

Lostndwoods wrote:

It’s not in my possession

Then how do you expect to unlock the iPhone?

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch, you will need to physically have the phone in your possession.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support