Vfrdtyky

If you knew that somebody had a knighthood, you would address them as "Sir Firstname", not just "sir".

"Sir" is still quite common as a term of address in the UK, but it would mostly only be used by an employee of a company to address a customer who's just some random member of the public. For example, a sales assistant at a shop or a hotel receptionist might address a male customer as "sir", but it would be unusual in, say,a business meeting.

It's not generally used between people of equal status in the situation. For example, if somebody drops something in the street and you want to attract their attention, it would (I think) be quite normal in the US to shout "Sir! You dropped your hat!", which would sound unusual in the UK. To use the terminology in the question title, if you're a regular man or woman, you probably wouldn't address another regular man as "sir".

For women, "ma'am" would be unusual in the UK (it's used to address the queen). The analogue to the American "ma'am" would be "madam". I have a feeling that it's less common than "sir" but I'm a guy so I'm not very confident about that assertion.

A warning: this answer refers to a regional dialect and definitely not to standard English. You're not likely to hear it as a learner as most people have manners and education enough not to speak in heavy dialect to outsiders, and it wouldn't be wise to try it out as it would probably be perceived as an error.

Colloquially, in parts of Ireland (including bits under the jurisdiction of the United Kingdom - for the nonce), it is definitely acceptable, even normal, to call 'regular men' sir.

In rural Ulster, one can call any man (and sometimes woman) of age sir and it carries with it no connotation of deference; it's used much as other regional Englishes use mate.

Due to the ethnic cleansing of the native Irish people and their culture from much of Ulster, and the settler colonisation of the north of Ireland with English-speaking Scottish lowlanders, to this day the English spoken in the north of Ireland is heavily influenced by the Scots dialect, and this sense of sir can be found in a Scots dictionary:

Scottish National Dictionary (1700–)

SIR, n. Also shir (Cai. 1891 D. Stephen Gleanings 82). Sc. usages:

1. As a common form of address between men of equal rank, esp. freq. among miners
   (Slg., Fif., Clc., Ayr. 1970).




2. Used in addressing a lady. Obs. in Eng. exc. dial. since 17th c.
   “The Highlanders use this term of respect indifferently to both
   sexes” (Sc. 1904 E.D.D.).

...

I presume this usage is also still current in parts of Scotland too (especially as the reference for the first sense in the above dictionary entry is dated 1970), although I couldn't actually say so myself.

Other than this very limited dialectal sense, in standard British English, putting knights aside (who, in any case, deserve no deference, in my opinion!), sir implies deference, status and hierarchy: it's used by shop attendants to customers, by pupils to teachers etc.

See, for example, the Oxford Living Dictionaries definition, a British English dictionary:

sir (also Sir)

noun

1 Used as a polite or respectful way of addressing a man, especially
one in a position of authority.

...

The existing answers do a good job of addressing the general case, but there's a particular situation which they don't mention and which is quite illuminating. When I was at school* in the UK in the '90s the standard forms of address for a pupil speaking to a teacher were "Sir" and "Miss". Some male teachers, particularly the younger ones, disliked being addressed as "Sir", although I don't recall that any of them came up with a suitable alternative.

To emphasise the point for clarity, even in a setting in which "Sir" was acknowledged as the correct form of address, it made some recipients uncomfortable. By extrapolation, in settings where there isn't a clear standard form of address it is likely to come across as overly formal.

(Some female teachers, particularly the married ones, also disliked being addressed as "Miss", but that's a separate issue).

* NB it was an independent school in Kent, England; experiences in other schools, particularly in other sectors or regions, may vary.

In the UK you should normally avoid using "sir" to address regular people. The use of "sir" tends to indicate that you serve someone (though there is no connection in meaning or etymology with serve and sir).

In British English men are not generally addressed as "sir". In the three examples you give it would be normal simply to omit the word "sir".

Hello, I'd like to speak to...

Thank you. Have a good day.

Good morning!

In none of these cases does adding "sir" make the expression more polite.

If you know someone's name you should use that in preference to "sir". Using "Sir" when you are on first-name-terms sounds particularly odd.

Thank you John. Have a good day.

If you don't know someone's name it is rare to address them at all. In a company, or at a social event, if you meet someone new, you can always ask for their name. If you are (for example) buying train tickets, you don't need to use the person's name, title or form of address at all. In 99% of normal interactions "don't use sir".

There are exceptions to the "don't use sir" rule: In many schools, and when speaking to a retail customer for example. In these situations there is a real sense of superiority (the teacher is superior to the student; the customer is superior to the shop-assistant). However many English Learners come from cultures in which not using the equivalent of "sir" is considered rude, and so tend to overuse "sir" when speaking in English.

There can be a class-based element to this, or at least there was, at one time. In Mary Crockett’s 1977 novel As Big as the Ark, a working class boy addresses an upper class man as “Mister”, and immediately apologises. The man was not knighted, he had no title. He was Mr Smith, not Sir John*. But still, the correct term of address for an upper class man when you don’t know his actual name is “Sir”; “Mister” would be appropriate for a man of the boy’s own social class.

This is less true these days; in fact it’s barely true at all any more, but vestiges of it can be seen when shop employees address customers as “Sir”.

* I cannot remember his actual name.

I for one address gentlemen as "Sir" when I wish to convey respect. Either the person has done me a favour, performed a task that I myself were not suited to perform or the person is simply older than I (or at least presents themselves that way).

It is a dying tradition and many stop addressing others in that way once they have left education. It is something that I do on a regular basis, but I rarely hear others do the same.