What causes echo on phone line?

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Tags

1. #caused
2. #returned
3. #wiring
4. #rather
5. #electric
6. #caller
7. #which
8. #delay
9. #voice
10. #transmission
11. #causes
12. #ending
13. #hybrid
14. #earpiece
15. #phone
16. #create
17. #signal
18. #called
19. #recipient