Tune Up Your Hookswitch
A phone's hookswitch should last for 50 years or more. If yours seems to be slowing down or causing static or both, you can rejuvenate it.
Remove the telephone shell. Remove the plastic dust cover from the hookswitch if there is one. Make sure the spring is still connected at both ends. Spray contact cleaner on the contact points. Exercise the hookswitch about five times.
Contact cleaner is available from RadioShack, home centers and online.
This diagram is for a wall phone. The desk phone hookswitch is similar.