Sure, they can get into the serving terminal, (pedestal) and hook their drop wire to yours. Call the phone company and report you hear others on your line. They will come out and remove the tap if there is one.

the other posters have failed to remember that when the telco runs a trunk cable, they run the entire cable the entire length of the run!that means, if you are house #1 on the trunk, your number will appear at ALL pedestals on that trunk just as house #50 will appear on the first pedestal. Your service maybe underground, but there are access pedestals all down the street as well as at the access point for your street.

if they access your line at the network interface device on the side of your house they can monitor or use your line... if you use 900 Mhz cordless telephones they can monitor your phone calls with a wireless scanner... if you use a wireless baby monitor - many (not all) can be monitored with a radio scanner... if there is a trouble on the line they may hear your conversations in the back ground on there line, or may have cross talk with your line... if they have knowledge of the telephone network, and the balls to open the main terminal serving your home, they can bridge the wiring at the terminal going into there home.... this is less likely for them to do, since it is physical evidence or proof that your line is purposefully and unlawfully bridged to a location other than your home...

probably call your phone company and have them check out your line for a tap or anything else