Check the cord itself and the connectors at each end. If there is even the slightest damage, the cord should be replaced. If you have to use a long cord, try not to use staples, nails, etc to fasten them. Lots of times this causes damage to the cord and you won't even know it. Try not to use splitters, which are designed to let other devices to share the same line.
Splitters often reduce the quality of a phone line signal.
If any other phone devices share this line it can introduce interference in your connection. If you need to plug more than one device into your modem line, install a wall jack with 2 outlets. Also don't plug into an electrical outlet that is designed to extend phone service into a room that doesn't have phone wiring. These jacks usually don't work for modem connections. Also don't run your modem line through any, surge suppressers or power supplies since these devices also can interfere with a modem connection .