Mac: How to record VoIP phone call (microphone and output simultaneously)?

The prime app for any task like this has always been Audio Hijack* from Rogue Amoeba - not cheap though for a single task, $49.

I've not tried it for that specific task type, but there's a demo available. Though it injects periodic noise, it should be sufficient to test whether or not it's fully suitable for your intended purpose.

Rogue Amoeba took over Soundflower after Cycling '74 stopped maintaining it, but it hasn't really been updated & doesn't have anything like the functionality of Audio Hijack.

Tweaked answer after OP edits.

You need to not route the mic directly to the speakers, or you will get immediate howlround.
You will always get some spill of the audio from the speakers into the mic, which may make it sound more 'reverby' - the only fix for that is to use a headset - but it will prevent the mic recording itself, playing itself, recording itself… …

See this example -

_{*No afilliation, just a satisfied user}