It depends on operator, BTS etc. and which codec it would negotiate with phone. Recently I noticed that in specific location 3G UMTS call quality is much worse

It depends on operator, BTS etc. and which codec it would negotiate with phone. Recently I noticed that in specific location 3G UMTS call quality is much worse than 2G GSM... (and there is that weird VoLTE, but it apparently requires specifically supported phone by operator with branded firmware..why!?)