I was wondering if it was possible to save on iCloud a WhatsApp call record.
I have an iPhone 6s.
The best solution is to put the call into speaker mode and use a voice recording app:
Then when you want to record the call, open your voice recorder app, start recording. Then also open whatsapp, call your contact and put the phone on speaker mode, this will then record the call at highest voice quality for you without any Automatic call recording apps.
Then you can add the file to iCloud Drive. On iOS you can access iCloud Drive in the Files app.