Voice calling has several issues.

a) it's a legacy product that operators don't care about anymore.

b) latency is important for voice, but modern voice calls have terrible latency. POTS was digitized (for T1/PRI) with g.711 which provides a 8kHz sample rate and 13 or 14 bit samples encoded down to 8 bit. PRI lines were multiplexed one sample at a time. There's a small amount of algorithmic delay, but not much.

Modern voice calling is packetized, and most systems have packets of at least 20ms, you have more encode/decode time, you probably have a significant sampling buffer, and you may have to wait for network access, so the receiver needs to add a jitter buffer to manage that. Additionally, echo cancelation needs processing time, and it's also harder with longer latency. Some systems use longer packet times to reduce network overhead.

c) codecs: g.711 has a terrible pass band, GSM codec takes the same inputs and compresses it harder. g.722.2 (amr-wideband) is better, but it requires proper call setup and that is problematic for calls from one carrier to another. If you're calling via apps, there's potential for more flexibility, but you likely get whatever the developer thought was good enough.

it depends on your carrier, I find voice quality to be great on cell calls, unless its verizon.