Members of Congress' offices can be reached through the U.S. Capitol Switchboard at (202) 224-3121. When you call, ask to be connected to the office of your legislator.
Be aware of timing. The most effective time to place a call to your Member of Congress is close to a vote or committee hearing. It is not required that you write a letter or have made an attempt to contact your member prior to a phone call, but it is helpful. If you have written or visited with your legislator in the past remind them or their staff.
Think carefully about the reason for your call before you pick up the phone. The goal is to clearly state what action you want your legislator to take. The call will be brief, so you want to know exactly what you're asking the legislator to do so you can get right to the point.
Identify yourself, make sure they understand that you are a concerned voter and tell the aide you would like to leave a brief message, for example: "Please tell Senator/Representative (Name) that I support/oppose (S.___/H.R.___).
Ask to speak with the aide who handles your issue. If you want to do more than leave a simple message, you want to make sure you're talking to the person who works on the issue. Usually, a staff member, not the Member of Congress, will take your telephone calls. Make clear your position and the action you think your member should take. Feel free to ask questions and share information about your issue, but be concise and considerate.