The general consensus (for the entire English speaking community) about the situation is at the moment in a lot of flux.
Historically (Early to 20th c Modern English), if you've seen or heard the person, then they present usually as one sex or the other (through clothes, name, or title) and you refer to them as either 'he' or 'she'. If you didn't know anything about the person, the default was 'he' (as prescribed in school and used by most people) or sometimes 'they' (Shakespeare and Jane Austen used this 'singular' they).
In the sixties (1960's) during second wave feminism, with genders becoming more common in roles that were by default the other gender (usually a female in a male role), it was becoming (more) apparent that assuming 'he' for an unknown was presumptuous and often wrong. So it became more common to see alternatives like 'he or she' or 's/he' or sometimes choosing 'she' instead of 'he'. In print, the singular 'they' (for unknown gender) was rarely used, but in speech people unconsciously and very naturally just say 'they' whenever gender isn't known:
"Someone just called me on the phone."
"What did they say?"
is entirely unremarkable and the way every one has said it. No one in the 20th or 21st c has said "What did he or she say?" ever.
Also, in the 60's (or even before) some people devised a number of alternative pronouns intending to be some form of gender neutral, but these never seemed to catch on in writing or speech, formal or informal. But if the gender was known, the classic corresponding pronoun was still used, 'he' or 'she'.
More recently (end 20thc, early 21st c), with both the advent of internet communication (where gender is more often not known or inferable) and the acceptance of people who don't identify entirely with the two genders, there's been quicker change, not always known to everybody. Though there has been more acceptance of the traditional singular 'they' (only for when gender is not known), there have been two further trends: 1) to use 'they' even when gender is known, and 2) to let people specify their own 3rd person pronoun (gendered, not gendered, a neologism, whatever it may be. So it is becoming more common to ask an individual which pronoun they'd prefer when others are talking about them. This is a fairly new situation (only the past few years). Because many people are aware of different habits but also many people are not so aware, there's a lot of variation in what people use (and also a lot of variation in what's accepted). Some human resources (HR) departments in some companies in the English speaking world have written strict behavior rules on what pronoun to refer to people, but this is not universal (nor even that common). An HR department is not a automatically what people are forced to do nor a reflection of what people actually do, but is more of a sign of general cultural trends.
This is a long explanation that doesn't necessarily directly answer your question because currently the situation is changing and different places do different things.
If you know absolutely nothing, 'they' or 'he or she' is currently most common ('s/he' doesn't seem to be used anymore in media). If you only know that they present as female, with a female name, and a female title (Ms.), it is most likely appropriate to refer to this person as 'she'. Few would find a problem with that. But if you find out that they prefer to be referred to as some other 3rd person pronoun, then common courtesy would suggest that you use this other pronoun.
To be clear, currently the great majority of people (more informal) and media (more formal) use 'he' for males and 'she' for females, and in the great majority of cases determining gender is easy (by first name, by appearance, or title).
Note: I'm attempting a descriptive answer, one that attempts to describe the history of what people actually do, and a minimum of what a language learner probably should do.