The toll free phone number for the U.S. Capitol switchboard is is: 1-800-361-5222 (90001). When you call the number, follow the prompts.
When asked for your 5 letter zip code, enter the number 90001. You will be connected to the main capitol switchboard. Just ask for the congressman or the senator you wish to speak to.
If you enter your home zip code, you will be connected directly to your Congressman or Senator.
You can call your senators for free, courtesy of this toll-free number set up by the tobacco industry. Just dial 1-800-343-3222, punch in any valid zip code, and you will be connected to your senator; call twice to reach both.