I hear that Webster’s dictionary has declared “They” to be the Word of the Year for 2019. It is now standard, according to Webster, to use “they” as a singular pronoun to refer to a person without suggesting gender. We can now say, “I told the caller to leave their number and we would call them back.”
To be grammatically correct back in the day, we used to have to say, “I told the caller to leave his or her number and we would call him or her back.” We could not say “they” because “they” is plural, and the caller is singular. Therefore “they” did not fit with one caller. Now, if we had a gender-free pronoun, like “it” we could have said “I told the caller to leave its number,” but we don’t use “it” for humans. Another complication.
The fact is that we have been saying “I told the caller to leave their number” for years, and no one minds except Boomers who were educated in the old school of English teachers who taught that “they” was plural and couldn’t be used any other way. Teachers like me. I remember haranguing my students over and over about matching pronouns with their antecedents in number and gender. I also remember the blank stares of incredulity they gave me in return. “What is this woman on about? Does she really believe the clap-trap she is spewing?”
It all comes down to your grammar attitude. Do you think grammar is PRESCRIPTIVE? Is grammar a rule book which you must obey to be an literate person? (Sort of like the Bible being a rule book you must obey to get into heaven). Or do you think grammar is DESCRIPTIVE? Is it a list of facts about the way real people use real language in real life?
Webster, in their wisdom, is choosing the Descriptive path by admitting “they” as a singular, gender-free pronoun. I say, “Well done. It’s about time.”