In theory, a SIP client which supports the OPUS codec can call another such device and the intermediate networks will pass the high quality audio end to end.

In reality, it's very possible that somewhere along the line, the call will be interpreted as a POTS (Plain Old Telephone Service) call, and therefore stripped of its fidelity.

Meanwhile, it's unrealistic to expect a phone call from a traditional telephone to sound like anything other than a phone call.