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What causes echo effect in telephone lines during phone calls? Does it mean that someone is spying on telephone line or? When I talk with some people, it happens, and with some of them not.

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    \$\begingroup\$ Spying shouldn't cause an echo, otherwise it would be too easy to tell. \$\endgroup\$ – jippie Aug 14 '15 at 19:55
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    \$\begingroup\$ These guys are using a crappy hands-free devices. So the mic is hearing the speaker. \$\endgroup\$ – Eugene Sh. Aug 14 '15 at 20:00
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If you're talking to someone using speaker phone then yes it is likely acoustic echo where their mic is picking up their voice echoed back from the room/ car or whatever.

There is also hybrid echo which is where the 2 to 4 wire conversion at the central office does not impedance match perfectly so you get some reflected echo.

It can be worse if there is already a lot of delay on the line such as in a VoIP system.

Here's a link that talks about the concepts.

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