>When is the last time a flagship phone release promoted its qualities as a telephone, like call fidelity?

Because nobody calls anymore. If anything calling is considered rude compared to texting, to the extent that some text first before calling.

>Others could better comment on this but I think landlines were leagues better in latency and sound quality.

landline latency (near instant, if you're both on the same CO) is basically impossible to achieve with a digital system, because of all the buffers and packets involved.