the process is simple. they get someone to flip on a house. a tip isn't enough for a search warrant, but with some surveillance they can easily obtain a warrant to tap the phone.
they locate where you're phone line is and they install a transmitter, which sends the signal to a truck, or a house near-by.... then they sit back and listen, and record every thing, every call, to where, by whom, and write every thing down....
the tell-tale sign is if you saw some people working on the utilities the last few days.... dropped calls, some static, interruptions, that sort of thing is possible....
get a pre-paid phone at walmart.... those are much more difficult to trace, and it's a practical impossibility to get them tapped. unless you're a special agent with the CIA or some shit.... but not with local law enforcement resources.....