Pick up the phone and ask the purpose of the call.
If they state it is regarding a debt, then simply tell them to send their formal collection ("dunning") notice, and terminate the call.
By answering the phone, you clearly establish an "initial communication with the consumer<" which then triggers their requirment to send a formal dunning notice within 5 days thereafter. That will let you know what the debt is alleged to be, and thus provide information on how to proceed, such as by way of a DV request or a pay for not reporting offer.
You gain nothing by ignoring them. You invoke a self-imposed requriment on their part to send dunning notice by picking up the phone, which will lead to gaining informed into on next steps.
