You might be confusing "do not know" for "do not care."

Yeah, my technique is to punch a random number into the machine, if it works then no problem, but if not the cashier will just scan the "store card" after the 2nd failure.

That's my technique too, but when the cashier asks to look up my number I give them 555-1212. That is what fed into my comment. Most people don't care what number is used, but many also don't know that 555-1212 is the secondary information number. This, of course, depends upon their age and geography. This comes up in the friendly conversations that result upon them looking up the phone number.

When I bought large items at crappy retailers like currys, the often refused to serve me without a postcode. I'd use SW1A 1AA, 2AA or 0AA

Except when buying a TV -- that would be W1A 1AA

Exactly.

Sadly the reverse lookup for 10 Downing Street is "Prime Minister and First Lord of the Treasury".

I told the cashier "That's me, don't you recognise me without the makeup?"

You should have said something like "note that the Chief Mouser to the Cabinet Office, albeit less known to the general public, also lives in the building", which is technically true.

Cashiers don't know, or don't care?

Of course some know but don't care, but the majority I met don't know.

How did you ascertain that they didn't know?

Because I was the one who provided the phone number and stood there talking with them.

How do you differentiate not caring what number you gave them from not knowing? The fact that they showed no reaction is more of a sign of how few fucks are given rather than everyone else's ignorance.

How do you differentiate not caring what number you gave them from not knowing

I said how in the comment to which you replied.

Where did I say, "fact that they showed no reaction"? Why do you interpret something unstated as a fact?

Because you failed to bother to give a more comprehensive answer when I asked how you knew I extrapolated. Did you personally conduct a survey?

Wasn’t 555-1212 the only 555 number that worked?

It used to connect you to directory information (as in "I need to number for mikestew in Seattle...") for the specified area code (or local, if no area code specified). I don't know if it still works or not.

There are other codes in there, caller & circuit id read-backs and other utilities the telco will use. They're usually private, but no serious harm if the public find out about them.