When I moved permanently away from my country of origin, I needed to cancel my private health insurance there. There was nothing on the website that would do it, so I supposed they wanted people to ring up. Just under a month before I finally left, I made the call. The first lady I spoke to was very helpful and although it took a while, we cancelled it and all was well. A week later, I got another call, explaining that although I had cancelled the insurance (which is paid monthly in advance), I had actually paid an extra month by mistake. I was busy, but the man assured me that it would be easy to refund the money to my bank account whenever I was free to call back.
When I called back, I got a different man, who suddenly informed me that it was not they who owed me money, but the other way around! He started taking me through steps to have me pay what I apparently owed. I felt uncomfortable about this, because I always pay these things promptly (hence overpaying without realising), and a few things he said sounded off. I said I was suddenly busy when he asked for my bank account details (they should already be in their system) and would call back to do it a bit later.
The fourth person I spoke with was confused--no, I don't owe them anything, they owe me. She sorted out my refund, and I made a point of complaining about the guy who had tried to make me pay extra. I had his name (when people on the phone introduce themselves I always write it down) so hopefully something came of it.