The Complete Guide to AI Voice Agents

An AI voice agent pipeline has four core components: (1) Speech-to-Text (STT) — converts caller audio to text using models like OpenAI Whisper or Google Speech-to-Text, (2) Language Model — processes the text, maintains conversation context, and decides actions using LLMs like GPT-4o, (3) Tool Calling — executes real-world actions (database queries, API calls, scheduling) mid-conversation, and (4) Text-to-Speech (TTS) — converts the response back to natural-sounding speech using ElevenLabs or OpenAI TTS. Modern systems like OpenAI's Realtime API combine these into a single WebSocket stream with sub-1-second latency. CallSphere uses both WebSocket (PCM16, 24kHz) and WebRTC transport depending on the deployment.

Single-agent systems use one LLM prompt to handle all tasks — simple to build but limited in capability. Multi-agent architectures (like those used by CallSphere) deploy specialized agents that hand off conversations based on intent. For example, CallSphere's healthcare system uses 1 agent with 14 specialized tools, while the real estate platform uses 10 specialist agents (triage, property search, mortgage calculator, viewing scheduler, etc.) with hierarchical handoffs via the OpenAI Agents SDK. Multi-agent systems excel when: (a) different tasks require different tools, (b) context windows would overflow with a single prompt, or (c) you need different safety/compliance rules per function.

Healthcare: AI answers patient calls, schedules across multiple providers, verifies insurance, handles prescription refills — HIPAA compliant with 14 function-calling tools. Salon & Spa: Fuzzy service matching, stylist preference tracking, upsell suggestions, loyalty/VIP management. Real Estate: Property search with vision analysis, suburb intelligence, mortgage/investment calculators, viewing scheduling. IT/MSP: L1 support automation with RAG (ChromaDB), ticket creation, password resets, SLA monitoring. Property Management: Maintenance dispatch, emergency triage and escalation, rent reminders. Sales: Batch outbound calling (5 concurrent), lead scoring, campaign management.

Production AI voice agents need: (1) Telephony integration — SIP trunks (Twilio) for PSTN connectivity, WebRTC for browser-based calls, (2) Database integration — agents must read/write real business data (appointments, tickets, orders), not just chat, (3) Analytics — post-call analysis including sentiment, lead scoring, intent detection, and satisfaction metrics, (4) Escalation — graceful handoff to human agents with full conversation context, (5) Compliance — HIPAA for healthcare, PCI-DSS for payments, GDPR for EU data. CallSphere deploys via Kubernetes with 3-7 day implementation timelines per vertical.