NAP (name, address, phone)
Your three contact details. What matters is that they say exactly the same thing everywhere they appear.
Google cross-checks your details between the listing, your site and the directories you appear in. When it finds three versions of your phone number, it trusts all of them less.
The usual mismatches are silly and very common: the street as 'Ave' in one place and 'Avenue' in another, an old number stuck in a directory from years ago, the name with and without 'LLC'.
It is one of the few local SEO items you fix in an afternoon and never have to touch again.
What to do about it
Search your business name in Google and check the first five listings that come up. Fix the ones with stale details.
Other words about the same thing