The cord to the hand set, or the hand set, need to be repaired or replaced. Take your handset cord and handset to a friend's home and try each part in their phone separately till, you learn which piece is causing the problem. Or, call your landline with a cell phone. Blow or say, "Five" drawing out the long i in five, "iiiiiiiiii" into your cell phone, while wiggling and twisting the cord where it goes into the handset, and into the phone base. Listening on your mobile cell phone, you might hear your voice start and stop while you are making a steady sound into the landline handset. That tells you the cord is bad, as that is the most typical problem. The cheapest thing is to replace the handset cord. But, you can get handy and buy the tools and RJ-9 plugs to repair your handset cord. The line cord into the wall needs an RJ-11 plug, but i assure you, that part is OK.
If the handset is the problem, replace the receiving ear piece element, or the whole handset. Although, you might get lucky and find a loose wire on the ear piece inside the handset.