It can be the basis for a motion to quash service or a motion to set aside a default judgment.
But you should not fool around or delay. If you have been sued and they claim you have been served, then if you do not file a response, they can get a default judgment against you.
You need to get a response on file asap.
$20,000 is a lot to risk by defending yourself. Go talk to a local attorney immediately.
I agree. If you don't believe the debt is accurate you need to defend yourself. Challenging service will involve much of your time and will only delay the inevitable. Plus, any default (automatic) judgment against you will include attorney fees etc. As my colleague has said, you really should get an attorney. Another option would be to contact the opposing attorney/party and come to an agreement on the amount owed and try to make payment arrangements.
I am licensed in California only and my answers on Avvo assume California law. The above answer is for general information only and is based on the information you posted. Every case is fact dependent, so to get a thorough analysis of your situation, you will need to consult face to face with an attorney licensed to practice in the jurisdiction where the incident took place. Do not conclusively rely on any information posted online when deciding what to do about your case. No attorney-client relationship shall be created through the use reading of this response on Avvo. You should never delay seeking legal advice, disregard legal advice, or commence or discontinue any legal action because of information in this response.