WhatsApp is just an internet-based app, all this proves is that you have an internet-connection, be that WiFi or mobile data. Neither your phone nor the mobile network treat it as a call, but as data usage.
Does your phone show network coverage? You ordinarily need that to make & receive calls. Are you using WiFi or the mobile network to use your internet apps?
Depending what model your phone is, it may have WiFi-calling enabling you to make & receive calls over any WiFi connection i that's what you're using.
