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It is legal. The initial letter is called a dunning letter. By law they had to have sent it within 5 days of their initial contact with you. That initial contact could have been that dunning letter. Maybe it got lost in the mail. Maybe you had moved. Mayve they are lying and they never sent it. In any case, if you were to ever go to court, they'd have the burden to prove that they sent it, which they probably cannot prove.
Never talk to a CA on the phone.
If this happened to me, I would wait a week to 10 days and assume they were lying about sending the letter out back in August. I'd wait for a new letter from them. If they don't send a letter, then I'd mail a DV letter CMRRR. If lacking any contact info, you could google the phone number. I'd also pull all 3 reports directly from the CRA and look for any hard or soft inquiries from "Debt Recovery". If they pinged your report, then you can lift contact info from that too.