Best Real-Time Speech-to-Speech AI APIs for Phone Calls (2026)

A Retell AI comparison of seven real-time voice AI options for phone calls distinguishes between native speech-to-speech models, cascaded speech-to-text/LLM/text-to-speech pipelines, and full phone-agent platforms that add operational telephony features. It identifies Retell as geared toward production call deployment through built-in phone numbers, SIP support, warm transfers, voicemail handling, testing, and post-call quality analysis, while OpenAI, Google, Amazon, Deepgram, and AssemblyAI generally require teams to build more of the telephony and escalation stack themselves. The comparison examines telephony access, interruption handling, tool use, human handoff, and pricing, while noting that it did not independently test latency or place calls. Gemini Live lists the lowest published audio pricing, OpenAI offers direct SIP options for its Realtime and Live APIs, and Deepgram and AssemblyAI provide flat-rate cascaded voice-agent services with transcripts that can aid debugging and compliance. The article advises choosing based on whether an organization prioritizes natural audio interaction, transparent transcription, predictable costs, direct phone-network integration, or responsibility for resolving failed calls, while acknowledging that Retell, the publisher, is among the products evaluated.

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