Makes no sense!

Try this one! My modem is right next to my phone so I could Un-Plug the phone line in on the modem and make phone calls.... Then later un-plug it from the phone and back into the modem! Worked for 2 yrs now if I plug the EXACT same phone cord now I get no signal but it works on the MODEM! This phone line ( I thought went dead ) ONLY works on the modem! IDEAS??