Remember that telephone calls are usually taken by a staff member, not the member of Congress. Ask to speak with the aide who handles the issue about which you wish to comment.
After identifying yourself, tell the aide you would like to leave a brief message, such as: "Please tell Senator/Representative (Name) that I support/oppose (S.___/H.R.___)."
You will also want to state reasons for your support or opposition to the bill. Ask for your senators' or representative's position on the bill. You may also request a written response to your telephone call.