What localhost doesn't tell you
A voice agent that works perfectly over localhost tells you the model and the application logic are sound. It tells you nothing about NAT traversal, TURN relay cost, or how the connection behaves on a mobile network with variable jitter.
The actual connection path
Direct connection via STUN is the fast path; the dashed TURN relay path is the fallback that only shows up on restrictive networks, which is exactly the class of network a demo laptop on office wifi never tests.