Get yourself locked up in the slammer for a night while carrying a fresh burner; observe them writing down your IMEI and IMSI; see if that makes you start getting robocalls.
It probably is today. There was a time when hospital admissions/discharges were published in the local paper.
As a population we used to be a lot less concerned with privacy. It was nicer in a way. It was a time when you didn't really worry about your identity getting leaked or stolen. If someone knew you were in the hospital, they might send you flowers or a card.
Get yourself locked up in the slammer for a night while carrying a fresh burner; observe them writing down your IMEI and IMSI; see if that makes you start getting robocalls.