Direct dialing Congressional and Senate offices will often reveal your location, but using the central congressional switchboards will supposedly not. You can also set your telephone to not reveal your originating number. There are many toll free numbers for reaching the Congressional switchboard. For example, a quick search turned up:

1-877-851-6437

1-888-335-3588

1-888-818-6641

1-800-828-0498

1-800-459-1887

1-800-614-2803

1-866-340-9281

1-866-338-1015

1-866-220-0044