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.
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.