To make a call, the user needs to have calling minutes from their respective service providers. On the service provider's end, VoIP calls are passed with the help of Softswitch. But for end-users, it's just VoIP Dialer, which they need to download from the Play Store or service provider's website and sign up with their service. Read this blog to get more information: https://www.revesoft.com/blog/mobile-voip/how-to-start-a-mobile-voip-business/