Re: cellphone, mobile, handy
"trousers is, essentially, a subset of pants."
Given that pants is an abbreviation of "Pantaloons", he's absolutely correct.
Quite _why_ the British universally equate pants as underwear is mysterious. Yes, I know "underpants", the question is how it got here from there and only in the last 40 years or so.
("Cellphone" comes from "cellular mobile telephone" -> "Cellular phone" in an era where non-cellular ones were visible to the masses (I had one, it was awful - but not as bad as WinCE phones). Euro-types called dumb mobiles "Handys" or "Handyphones")