For me, no such thing normal or Whatsapp call is clearer. Nowadays, both normal and or Whatsapp call is using digital or packet data already. If either signal is bad, then the other one is better (at least).

The problem with Whatsapp call or any other apps call I think is telco probably did not give QoS on voice data. On telco normal line, the voice will get priority over other data. So, less delay if any due to other data transmission.

Me.... hate voice msg and I think never call anyone through Whatsapp