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On the other hand, if the debt is not yours, it's possible spammers/scammers have happened to target both your old and new phone number blocks. It's also possible that the true owners of that debt are also frequently changing their phone numbers and you happened to receive one of their old phone numbers.
If the calls are legitimate but the debt is not yours, you might have to resort to contacting the collections agency to inform them the debt is not yours (see here).
Hope this helps 😀