If you use Bluetooth headset, you probably have bad quality of voice in VoIP calls via Skype and other apps. This happens on Linux and on Windows too.
This happens because when an application starts a call - it create a recording stream with property media.role=phone (telephony applications should set that property for their streams), and PulseAudio switch profile of your Bluetooth Headset from A2DP (Stereo high-quality one-way channel) to HSP/HFP (Mono low-quality 2-way channel for voice-only).