Try the following possible solutions:
A) Replace the handset cord (curly cord between the telephone base and the handset) with a known good one. (The easiest place to find a “known good” part is from a telephone that is working.)
B) Replace the handset (piece you talk into and listen from) with a known good one.
C) Replace the base cord (cord between the telephone and the wall jack) with a known good one.
D) Replace the telephone base with a known good one.
If you have completed these steps and still have a problem with the telephone, there must be a more serious problem that requires a technician. You should contact your telephone vender at this point.
When calling for assistance, always remember to leave your business telephone number where you or someone you appoint as a contact person on site may be reached.