A few years ago, I signed up for an Apple Watch eSIM plan (which is a special type of eSIM plan that Apple make cell carriers agree to offer as an add-on to a normal cell subscription for your iPhone, and there is no other way go get an eSIM for it).

I then started regularly receiving phone calls (to my iPhone) intended for someone else. At first I thought it was a wrong number or an old number and kept telling them to remove this number. Buy the calls kept coming and I eventually I dared to ask what number they had dialed. And it wasn't a cell number I recognised.

After contacting support for my carrier, what I figured out was that the Apple Watch eSIM has its own phone number, for some reason, but it's not one you're supposed to know about; as an extension of your phone's subscription, the Apple Watch eSIM notionally has the same number as it. But they were calling the secret number associated with the eSIM, somehow. And I think there was a problem in the number routing table somewhere, because I think this number may have been in use with another cell carrier, and the calls only went to me when calling from my network?

Absurd nightmare situation.