Volume I

Engineering notes on AI phone calls.

Long-form deep-dives on voice latency, telephony, evals, and everything we learn while building the API for AI phone calls.

• No. 04·April 25, 2026·6 min read

  The best Vapi alternatives for voice agents in 2026

  A side-by-side of CallingBox, Retell AI, and Vapi across pricing, latency, turn-taking, telephony, and compliance: the criteria that decide whether a voice agent ships to production.

  Jonathan Chavez·Co-Founder→

• No. 03·April 24, 2026·9 min read

  Best Way to Build a Voice AI System

  The best way to build a voice AI system is one integrated real-time stack. See the architecture, tradeoffs, and how to launch a CallingBox agent in minutes.

  Jonathan Chavez·Co-Founder→

• No. 02·April 24, 2026·10 min read

  How to build an AI cold caller that books meetings

  Building an AI cold caller is a system architecture problem, not a prompt problem. The four hard layers (answering machine detection, dialing strategy, latency, and CRM data dips), plus a build-vs-buy framework.

  Jonathan Chavez·Co-Founder→

• No. 01·April 23, 2026·11 min read

  How to build an AI phone answering service in 2026

  Two paths to an AI phone answering service in 2026. The DIY stack: six layers, four to six vendors, weeks of carrier paperwork. The CallingBox path: one agent, one number, one webhook.

  Jonathan Chavez·Co-Founder→