[modeless]

Maps business listings are publicly editable and continuously updated by Google from any source they can find. Especially phone numbers. Google employs people to search the Internet all day long for business phone numbers to update on Maps. You published the number publicly, specifically listed as contact information for your business, on a Google site even. There is no mystery here.

[Dan-Q]

Could be! My only counterpoint is that every time they've previously made a change, they've emailed to say so, and I've searched my email (including spam) and not found any such mention this time around. /shrug/

[modeless]

You still need to update your post to include the obviously relevant information that you explicitly gave Google this phone number for the specific purpose of publishing it as public contact information for your business.

[JKCalhoun]

Only a reminder to never give out your phone number, I suppose.