We Tested 4 Text-to-Speech Engines on 12,000 Live Healthcare Calls — Here's Which One Patients Actually Trust

Last quarter, we ran our production voice AI receptionist — Loquent — across four different TTS engines simultaneously, split-testing real patient calls at dental and healthcare clinics. The results surprised us: the most "natural sounding" engine in demos performed the worst with actual patients. Why We Ran This Test At Autor, we've been running Loquent in production for over a year now. It handles thousands of automated calls per month for healthcare and dental clinics across Canada — booking appointments, answering insurance questions, handling after-hours triage. The voice is the product. If patients don't trust the voice, they hang up, and the clinic loses a booking. When we first built Loquent, we picked our TTS engine the way most teams do: we generated a few sample clips, played them for ourselves, and went with the one that sounded best in a quiet office. That worked fine until we started digging into our call analytics and noticed something weird. Our completion rate — the percentage of calls where patients actually finished the full interaction instead of hanging up or asking for a human — was hovering around 74%. Good, but not great. We suspected the voice itself was part of the problem. So we designed a proper A/B test. Not a demo comparison. A production comparison on live calls. The Setup We tested four TTS engines across 12,247 calls over 8 weeks. Each engine handled roughly equal volume, randomly assigned at call start. All other variables stayed constant: same prompts, same Anthropic Claude backbone for conversation, same Twilio infrastructure, same clinics. The four engines: Engine A : ElevenLabs (Turbo v2.5) — our existing production engine Engine B : OpenAI TTS (tts-1-hd) — the model most teams default to Engine C : Deepgram Aura — optimized for real-time, low-latency use cases Engine D : A newer entrant we'd been evaluating (under NDA, so I can't name it) We measured five things: Completion rate — did the patient finish the full call flow? Time