KX-T: Idiot post of the day: What is extension 198 and why is it answering Line2?

[Tom Stewart]

I have a KX-TD308 at home, with generally one line connected most of
the time. Occasionally, though, I pop the SIM from my mobile into an
adapter so I can use my mobile # as Line2. Today, I did so, but
incoming calls are getting answered automatically without ringing any
extensions. The call log is showing that extension 198 is answering.
My extensions are numbered in the 10's and 20's and phantoms in the
20x range. VM is 295/296.

What feature have I accidentally setup??

Thanks for any help!