Debt collectors are required to validate the debt and send you verification within five days of their initial communication with you. The Fair Debt Collections Practices Act (FDCPA), section 809 (Validation of Debts), subsection a) lays this out clearly:
a) Within five days after the initial communication with a consumer in connection with the collection of any debt, a debt collector shall, unless the following information is contained in the initial communication or the consumer has paid the debt, send the consumer a written notice containing:
- the amount of the debt
- the name of the creditor to whom the debt is owed
- 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
- a statement that if the consumer notifies the debt collector in writing within the thirty-day period that the debt, or any portion thereof, is disputed, the debt collector will obtain verification of the debt or a copy of a judgment against the consumer and a copy of such verification or judgment will be mailed to the consumer by the debt collector
- a statement that, upon the consumer's written request within the thirty-day period, the debt collector will provide the consumer with the name and address of the original creditor, if different from the current creditor.
However, if you notify them in writing to dispute the debt, the debt collector has to stop collection of the debt and send you written verification of it (which they already should have anyway). Per the act:
b) If the consumer notifies the debt collector in writing within the thirty-day period described in subsection (a) that the debt ... is disputed, or that the consumer requests the name and address of the original creditor, the debt collector shall cease collection of the debt ... until the debt collector obtains verification of the debt or any copy of a judgment, or the name and address of the original creditor, and a copy of such verification or judgment, or name and address of the original creditor, is mailed to the consumer by the debt collector.
c) The failure of a consumer to dispute the validity of a debt under this section may not be construed by any court as an admission of liability by the consumer.
As for debt collectors that don't follow the act, section 813 (Civil Liability), lays out the consequences clearly. I won't quote it because it's a little long, but it's important to realize that per the title, these are civil penalties. That means they normally require a suit to be filed against the debt collector; if you read this section of the act, you can readily see that the penalties will occur as the result of civil legal action.
In short, yes, the debt collector is required to send you verification within a short time period after initially informing you. If they don't, you need to talk to a lawyer if you feel civil action is the best recourse. Since I'm not a lawyer, I can't, and don't, pretend to provide any meaningful legal advice.
Side note: I should add that one criticism of the FDCPA is that it limits the civil penalties the court can award an individual, above and beyond the damages caused by the debt collection agency, to $1,000. The act was passed in 1977, so inflation has reduced the real amount and purchasing power of that award.