Well, that’s what the aforementioned
org.nemomobile.voicecall.VoiceCall.silenceRingtone
should be doing. But it doesn’t seem to work at all.
The only other idea that comes to my mind would be doing things the reverse way, i.e. disable the ringtone completely for all calls and only enable it for whitelisted / not blacklisted calls when they come. But that would be risky, because if the command to enable the ringtone somehow didn’t work sometimes (and as we can see, many things do not work as expected here), important calls could be missed.