Back in 2017-8 I've got a four consequent top ups of an equivalent of $50 each (so a total of $200) from an unknown source. After that I've got a call from a resident cell number saying I've got a $200 fine for watching porn that I laughed off and hung up. I thought it was a scam to make me pay the "fine" with the money I've just got and then call the operator, tell them they "mistakenly" paid for the wrong number (four times for $50, lol) and get the money back. So I sat there and waited for the money to be recalled... but it never happened.

My theory is that it was exactly then I was changing operators while keeping my number, so the scammers tried complaining to the wrong one and failed. Not that I had any objections for the 2 years of free calls and data this got me but this still is a bit of mystery to me.