When you dial 202-224-3121 you are directed to an operator at the Capitol switchboard. This switchboard can direct you to both senators as well as representatives.
Once the operator answers, ask to be connected to whomever you are trying to reach. They will send you to your senator's or representative's office line, and a legislative assistant will answer the phone.
You can also find the direct number to any member's office in the Senate phone list or House phone list. Don’t forget that you can also call their local district offices, or even stop by to have a conversation in person!