There are quite a lot of occasions I experienced intermittent quality problem when making or receiving WhatsApp voice call. The sound from the other party just couldn't transmit well to my phone.
This kind of intermittent situation occurred even when I was connected to 5 GHz WiFi with high speed fibre Internet access, and I was very sure that the network utilization during the call was low without any congestion at all.
After some trial and error attempts to improve the situation, I finally found out the solution to fix this WhatsApp voice call intermittent problem.
You just need to change one setting in WhatsApp, and your WhatsApp voice call will be magically turned from intermittent to very clear.
If you are also experiencing similar problem with WhatsApp voice call, try go to Settings > Data and storage usage > Call settings and enable "Low data usage".
I am not sure what does WhatsApp do behind the scene with this setting turned on, but it does magically fixed my voice call problem. It works both when using WiFi or using mobile data.