Find phone
How can I find my second phone that is on my account?
Question marked as
On what account?
You can use the Find My iPhone App to track any device that is logged in to iCloud using your Apple ID.
If its not using the same Apple ID as your other iPhone, then you would need to log in to the Find My iPhone with the Apple ID that is being used on the lost iPhone for iCloud. Only log in to the App, nowhere else.
See here: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support
Being on your same cellular service account has no bearing on tracking it.
