Reset a replacement iPhone

Erase all Content and Settings, would be done on the device you replaced if possible, not on the replacement you received.

The Replacement, should have had nothing on it to be erased, as it likely was never setup to begin with, so Erasing all content and Settings would not be necessary for it.

You can always Erase all Content and Settings at any time from the Settings ➜ General ➜ Reset ➜ Erase All Content and Settings.

When you set up the replacement phone......did you miss the step to transfer data from a backup of another phone over to the replacement phone?

If yes, you can erase the replacement phone and then start over again with a new setup from the "hello" screen and then choose the option that you need during the setup of the replacement phone.