The person on/at the other end of the line

The person on/at the other end of the line
 

What do you call the person on/at the other end of the line (=telephone line)? An opponent?

We usually just say 'the person on the phone' or 'the person on the other end of the line' if we aren't providing the person's identity or relationship to us.

'Opponent' would not be used.
'Interlocutor' may be used, but this isn't a common word.

Thank you.