Maybe the problem occurs when using Bluetooth/BT.
Possible fix: Developer options, switch off “Bluetooth A2DP hardware offload”.
On my OP6 when using BT to play music, I have to disable BT before a call and even than only sound on Speaker.
Or first switch off Bluetooth and restart. Don’t use BT at all.
(The same bug as in LineageOS21 for OP6)
In iPhone there is an option for calls to have sound through the phone speaker as default.
Calling via whatsapp is working.
Echilada/OnePlus6 with U build.