It depends on the setup you have. Most VoIP app-to-app calling is always free. Though when calling mobile or landline numbers, it may cost you a little.
Other VoIP services like business providers or Hyperoptic cost a monthly fee. You can find many consumer-level apps on the App Store or Google Play to make free VoIP calls.
Even when you use premium or Voice Over Internet Protocol services for business, the charges are significantly low compared to traditional landline charges.