Building a remote MCP server that makes phone calls: long waits, mid-call questions, getting paid
jakemarsh · reddit · 2026-11-14
The author built CallForMe, a remote MCP server letting agents call businesses (hours, quotes, reservations, hold queues) and shares hard-won engineering lessons.
- Long waits vs short tool calls: placecall returns a callid instantly; getcall long-polls with waitseconds (up to 50). Stateless Streamable HTTP, no notifications needed.
- Mid-call questions: businesses' questions surface as pendingquestion on the next getcall; the agent answers via answerquestion, spoken into the live call, with a 45-second fallback.
- Auth & payments: OAuth 2.1 with dynamic client registration and PKCE; guest accounts with no-screen redirect when no browser session exists. Payment comes as needssetup with a link — but ChatGPT hides output until its turn ends, so the agent must end its turn on the link; the server auto-places the call 20s after payment since users rarely report back.
- Tool-name guessing: unknown tools now return the closest real name plus the full list, and argument errors state what to send next.
A demo number is free to try; real calls cost $1 for 2 minutes then $0.40/min, nothing if unanswered.
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