You can make internet-based calls (WhatsApp, Telegram, Messenger, FaceTime, etc.), but you cannot make emergency or cellular calls (112/911) using an eSIM data-only plan.
Data Credits, which are displayed as Credits are your prepaid balance on saysim.io. Instead of entering payment details every time you buy an eSIM plan, you can add Credits to your account and use them whenever you need. One Data Credit always equals 1 EUR, and the balance stays in your account until you spend it.