"They" used to be the common gender neutral singular pronoun, and still is colloquially, but English lacks a universally accepted word for this. Here are some contenders.
Someone left their phone behind.
This is common in speech, but avoid it in formal writing because "their" is not truly singular. It is the most gender neutral of these options because it isn't gender binary.
Someone left his phone behind.
I would not go so far as to say that it is offensive to default to the masculine pronoun, but is is a bit insensitive. This still has widespread acceptance, but it's definitely losing popularity.
Someone left her phone behind.
In an effort to draw attention to the peculiarity of using a masculine pronoun for an unknown individual, some people do the same with feminine pronouns. This is often meant to make you pause to reconsider your preconceptions, and people also use it as a reminder to themselves.
Someone left his/her phone behind.
A little jarring, but singular and neutral. Also consider her/his if you want to make people pause.
Someone left his or her phone behind.
This blends in better with text and speech than the slash, is grammatically correct, and is more politically correct than just "his."