I've gone through this issue myself as well, until I happen to buy a SteelsSeries Arctis 7 headset that comes with a chat audio input which you can record as a separated audio track. I have set Discord audio output source to the Chat channel of the headset and voilà, I can record voice-calls on a different track.
I'm not saying to buy THAT one in particular, because hot damn, it's expensive, but having a headset with game and chat mix will be your best friend. Or an alternative could be having one USB over-ear headset and then plug some earbuds in the green 3.5 jack port behind your PC. You set the chat to come out from the earbuds, then the rest on the headset and then you set OBS to record those audio sources on different tracks.
Hope this helps with your issue!