These four advertise time-to-first-audio between 40 and 200 ms, and an independent harness measures their cloud medians at 188 to 313 ms — but the number that breaks a phone call is the spread, not the median, and one vendor's jitter is nearly four times another's. Price moves about 2.5× across the field and predicts neither. The tail is bought with deployment.
Word error rate is close to settled between the four, and a couple of points of it lands on words your intent classifier ignores. The slice that decides whether a voice agent feels human is end-of-turn detection — several times larger than the transcription latency beneath it, and the one thing the four providers genuinely disagree about.
Google's shopping agent now phones local shops to check stock — a channel that carries none of the signed identity, scoped authorisation, replay protection or verifiable receipts that AP2 and its rivals were built to provide. The phone is not a stopgap on the way to universal protocol adoption; it is the permanent floor of agent commerce, covering the merchant tail that will never implement an API, and it has no trust primitives at all.
Voice is now the interface most agents will spend the most time in — and four platforms have made architecturally opposite bets on how to wire speech, language, and tool-use into one round-trip. The right pick depends less on TTS voice quality than on whether you control the audio path, the model, or just the prompt.