The FDCPA says:

(3) a statement that unless the consumer, within thirty days after receipt of the notice, disputes the validity of the debt, or any portion thereof, the debt will be assumed to be valid by the debt collector;
So if you can prove that you did not receive the notice, then the 30 day clock does not start until you do receive the notice. The burden of proof lies with you. Do not talk to them on the phone. In writing only, otherwise proof is much more difficult.