Do companies really call your old employer and ask about your employment there?

As part of the onboarding process of my new company is to do a background check. Do they really confirm the reason of your resignation? or the reason why you left your previous job?

Do they check that deep? I somehow didn't told the HR of this new company the real reason why I decided to leave my previous work. Kinakabahan lang ako if iverify nila yun sa old employer ko and di mag match sa sinabi kong reason