> I unplugged my landline phone. I have only plugged it back in once, and briefly, when debugging my internet connection, to check whether the line had a dial tone.

That's generally the only thing landline phones are good for anymore. When I lived in the UK I was with the nerdiest ISP there (AAISP) and they offer a "broadband only" landline, that carries your DSL and a basic diagnostic service including a dialtone (mostly to prevent wayward OpenReach engineers from misdiagnosing your copper pair as "unused"), but no real phone calls. Suited me fine.