Facetime audio takes up SSD space

I was just using FaceTime audio and my phone told me I was running out of space. I was curious as to how this was possible as I knew I have about 2GB free yesterday. I kept going back in and checking space whilst on the call and it eventually got down to 0 bytes free. Once the call was over, it went back up to 2.4GB.

Given that RAM and SSD space aren't wildly different things, it seems possible that FaceTime would overflow it's RAM usage onto the SSD.

Does anyone know if what I'm observing is true? Is FaceTime really using many gigabytes of my iPhone's storage when its operating? If this is true, why is FaceTime buffering so much to disk like this? And is the buffered content encrypted in anyway?