can't find my iPhone with ringer off

If that other iPhone of yours is on the same Apple Account or Family Sharing, then open Find My, select your lost iPhone in Devices and find it from there. If not, you can sign onto iCloud.com/find with your Apple Account credentials and find your iPhone there. By the way, the Find My sound plays even if the device is in silent mode or the volume is muted.

Are you using the Find My app to locate your phone, or are your attempting to make a call from the other phone to your phone? If using the Find My app your phone should still make a sound whether the ringer is ON or OFF.