If it also affected internet, then instantly. WFH relies on it. Otherwise, probably a day or two of not getting a call from either of my adult kids would make me start to wonder.
We have a minor communication error. In my mind, I consider "calling others" to mean to carry on a (desired) conversation with other people (back and forth dialog). I only use the phone for calling doctors, arguing with insurance companies, and such. There is no "dialog" there, just voicemail. (To hear us laughing at you, press 9). My apologies for the misunderstanding.
I'd have to hear it on the news or hear people complaining their electronic toy had suddenly done U/S. But one reason I don't use Facebook - subscribers are obliged to provide their cell phone number - I can't honestly do that, because I don't have a cell phone.
If only cell 'phones - or "mobile" as portable 'phones are incorrectly called here in the UK - were off, I may not notice if for some while because I use the landline and Internet for most of my communications.
If the Internet and broadband too, then I'd notice it very quickly.
Shuhak 