Basically, any piece of equipment plugged into your phone jacks can act as an antenna and pick up CB or AM/FM broadcasting station frequencies -- creating noise on your line.
Cordless phones are notorious for picking up other people's conversations as well as radio signals. If you have a cordless phone, read these consejos para solucionar problemas to figure out what you can do.
If you don't have a cordless phone, do you have a satellite TV receiver, computer modem, answering machine, caller id box, fax machine or alarm/security system? If so, any (or all) of them could be interfering with your reception.