GSM was indeed heavily compressed (voice channels were only around 12 kbit/s, compared to 56-64 kbit/s on landline), but comparing a modern VoIP codec like Opus (which is what almost every VoIP solution uses these days) to GSM FR or even EFR compression is like complaining about 64 kbps MP3 compressed by an early 2000s codec sounding bad and going back to vinyl for quality.