Hello,
There's no such thing as a silent mode nor any ringtone volume setting in the Calls section (nor any other one actually) of WhatsApp notifications settings.
There's only the ringtone currently in use (apparently, the same as the current phone ringtone, a choice for an external app to be able to change it appears when pressing on the "Ringtone" line) and a vibration mode setting offering four choices (from "Disabled" to "Long"), and that's all.
WhatsApp definitely seems to be using the current "Media" volume rather than the "Phone" volume, EXCEPT, as Saabina first explained in the original post, when the phone is NOT locked.
As a matter of fact, I always keep the "Media" volume disabled so that I don't bother people around while checking social media or playing games, because noisy ads tend to pop up without notice (plus their own sounds are unwanted as well).
As a result, that's only when WhatsApp's would ring loudly on incoming voice calls (still with the media volume completely off, but of course the phone volume on). However, as mentioned above, it's completely mute when the phone is locked, so it only vibrates, and as a result, I miss incoming calls most of the time.
As for the Call notification section in the general phone settings, everything is enabled normally, so that's not the problem.
I think the actual problem is exactly like Saabina described : incoming voice calls in WhatsApp only use the Phone call volume when it's unlocked, and seems to be using the Media volume instead when it's locked.
Which is utterly stupid IMHO, and would need to be changed, but can most probably only be done by WhatsApp themselves and not by Samsung.
HTH
