Was accused y/day by an irate guy on my mobile, that he had found me using his wi-fi, I assure you I was not. How had he obtained my Mobile number to call me and could he obtain anything else from my laptop while I was using it. I would not have a clue if anybody was using my wifi, I thought you had to know the pass-code to use wifi that belonged to someone else.If I want to use it at my daughters they have to give me the code.
Like you say, a password is normally needed to use a WiFi network, and there's no simple way to get a mobile number for a device connected to your network.
Id be logging date/time/content of the call, in case he calls again. If so just state clearly you are not using his WiFi and not to call you again. If he harasses you any further I would be chatting to PC Plod about it. Sounds like a lead in to a scam to me.