PSTN / SIP

Sierra is the customer-service AI employee for large consumer brands: it resolves support conversations across chat and voice, takes actions in business systems, and bills on outcomes.

Decagon builds autonomous customer-support agents that resolve chats, emails, and calls for mid-market and enterprise brands, with engineering-grade tooling for the ops team behind them.

Vapi is the developer platform for voice agents: API-first orchestration of STT, LLM, TTS, and telephony so you can put an agent on a phone number in minutes — and swap every layer.

Bland AI is the enterprise phone-agent platform: autonomous inbound/outbound calls at scale with an emphasis on security, compliance, and call-center-grade throughput.

Retell AI is a voice-agent platform focused on production call quality: fine-grained latency tuning, interruption handling, and both API and no-code agent building for phone agents.

Synthflow is the no-code voice-agent product: agencies and SMBs build receptionist, intake, and outbound agents from templates without touching telephony or APIs.

ElevenLabs Agents is the voice agent platform built on the industry's best-known TTS: conversational agents with top-tier voices, phone and web deployment, and model-agnostic reasoning.