One MCP server. Any call provider. Fully local, fully hosted, or bring your own frontier model.
CallMCP is a single Model Context Protocol server that defines one tool contract — 14 tools, identical schemas, identical error shapes — for outbound/inbound telephony: calls, SMS, recordings, transcripts, and phone-number lifecycle. Point it at a hosted backend, a fully local backend, or a bring-your-own-key composed backend, and your agent code doesn't change. What changes is which tools are present (via capability-gated dynamic discovery), never the shape of a tool that's there.
The normative reference is SPEC.md. This README is the pitch; the spec is the contract.
Before connecting a real provider, run npx -y @callmcp/server doctor.
For a safe local walkthrough, run npx -y @callmcp/server --sandbox.
The server fails closed when no driver is configured; the mock driver is only
available through explicit sandbox configuration.
See examples/ for ready-to-paste Claude Desktop / Claude Code configs for each of the three legs below.
Three structural claims
This isn't "a Twilio wrapper with a marketing page." Three things are actually built into the contract, not bolted on:
1. Provider-neutral, multi-backend contract
Every driver — hosted, local, or BYOK — implements the same 14 tools defined in SPEC.md. Driver-specific behavior lives exclusively inside a namespaced options.<driver_id> passthrough object. There is no make_call_twilio or make_call_kaicalls. A tool either exists for your configured driver (and works, fully, per spec) or it's absent from tools/list — never present-but-broken. Swapping your call backend is a config change, not a rewrite.
2. x402-funded autonomous provisioning to production
search_numbers, buy_number, configure_number, and list_numbers are never human-approval-gated — they don't contact a third party, they contact your own provider account. That includes settling cost via x402 machine payments when a driver requires prepayment (INSUFFICIENT_FUNDS embeds a full x402 challenge an agent can settle and retry). An agent can search for a number, pay for it, configure it, and go live — autonomously, in production, with real money — without a human clicking anything. This is a deliberate contrast with vendor MCPs whose autonomous/self-serve paths are trial- or sandbox-scoped only.
3. Human-in-the-loop outbound approval as designed consent architecture
make_call and send_sms — anything that contacts a phone number that isn't the calling agent's own infrastructure — structurally require a valid approval_id or a standing allowlist match before they act. This isn't a policy suggestion layered on top; it's enforced in the server core, so no driver implementation can bypass it. Primary path is MCP elicitation; the fallback for non-elicitation clients is a single-use out-of-band approval URL — the gate is designed to never silently block forever and never silently open. Read as a TCPA-consent posture: the contract makes "who approved contacting this number, and when" a first-class, auditable object (request_call_approval / list_approvals), not an assumption baked into application code you have to get right yourself.
CallMCP vs. single-vendor MCPs vs. DIY
| Capability | CallMCP | Vapi MCP | Telnyx MCP | AgentPhone MCP | DIY (Twilio + your own glue) |
|---|
| Tool contract | One schema, 14 tools, portable across backends | Single-vendor, Vapi only | Single-vendor, Telnyx only | Single-vendor, AgentPhone only | No contract — you write and maintain it |
| Swap call backend without rewriting agent code | Yes — driver swap only | No | No | No | No |
| Self-hostable / fully local option | Yes (Dograh driver) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | Yes, but you own the entire stack |
| Bring your own frontier model as the call's "brain" | Yes (BYOK driver: any transport + any LLM) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | Yes, fully manual integration |
| Autonomous, funded, production-scoped number provisioning (x402) | Yes — search_numbers/buy_number/configure_number ungated, x402-payable | (unverified, recheck before publishing) | Public self-serve docs read as trial-scoped autonomy, not funded production autonomy (unverified, recheck before publishing) | (unverified, recheck before publishing) | No — manual console purchase, manual compliance |
| Outbound approval / consent gate structural in the schema | Yes — make_call/send_sms require approval_id or allowlist match, enforced server-side | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | No — you build and maintain your own gate |
| Standalone SMS (not agent-mediated only) | Capability-gated per driver, honestly reported — never claimed where it's actually agent-mediated | (unverified, recheck before publishing) | Yes, native | (unverified, recheck before publishing) | Yes, native Twilio API |
| Call recording | Capability-gated per driver, honestly reported | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | Yes, native Twilio API |
| Real-time transcript streaming | Capability-gated per driver, subscribable resource when supported | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | DIY — you build the capture/assembly layer |
| Source availability | MIT-licensed server + Apache-2.0 spec, public monorepo | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | N/A — it's your own code |
| Markup over underlying carrier cost | Zero in the OSS server — it routes at cost; KaiCalls hosted tier is an optional convenience layer, not a requirement | (unverified, recheck before publishing) | (unverified, recheck before publishing) | (unverified, recheck before publishing) | Zero markup, 100% of integration cost is yours |
Every "(unverified, recheck before publishing)" cell is a placeholder, not a claim — we do not put a number or a fact about a competitor's product in this table without a citation behind it. If you're reading this before that recheck has happened, treat those cells as unknown, not as "no."
Who builds this, and what's actually free
CallMCP is built and maintained by CallMCP / KaiCalls. Full transparency, because hiding this costs more credibility than stating it:
- This OSS server is not a loss-leader funnel with the real functionality locked behind a paywall. It routes to any configured driver — hosted, local, or BYOK — with no markup and no artificial capability gating. A self-hosted Dograh driver or a BYOK Twilio+LLM driver gets the exact same 14-tool contract as the KaiCalls driver.
- KaiCalls is the hosted default — the path of least resistance if you want a call backend running in minutes with nothing to self-host. It is not the only supported path, and the spec and conformance suite exist precisely so that claim stays true as the driver ecosystem grows.
- The driver interface is public and the conformance suite runs in CI. Any driver — including ones we didn't write — can claim conformance, and community PRs are held to the same bar KaiCalls' own driver is held to (see
SPEC.md §6).
If any of the above stops being true, that's a bug in this README, not a hidden asterisk — open an issue.
Driver capability matrix
Capability is expressed by presence, not by runtime errors (see SPEC.md §2.2). This table mirrors the spec's degradation appendix (SPEC.md §7) — the honest state of the wider backend landscape at spec-writing time (2026-07-09), not a ceiling on what CallMCP itself can do.
Launch drivers (this repo)
| Capability | kaicalls (hosted) | dograh (local) | twilio_openai (BYOK) |
|---|
make_call / end_call (hangup) | Yes — hangup via Vapi controlUrl, treated as call-scoped state | make_call yes; no external hangup endpoint (supports_hangup: false) | Yes, native Twilio call control |
search_numbers / buy_number / configure_number | Yes | No purchase flow — BYO carrier account (supports_number_purchase: false) | Yes, native Twilio number API |
send_sms | Capability-gated on the underlying Vapi-class backend; not claimed unless a real standalone send path exists | No SMS capability | Yes, native Twilio SMS |
get_recording | Yes, once wired | Capability-dependent on the local stack | Yes, native Twilio recording |
get_transcript / realtime streaming | Yes | Yes, baseline (Dograh's GET /{workflow_id}/runs/{run_id} returns transcript_url — the one fully source-verified transcript path in the wider landscape) | DIY assembly from realtime session events; supports_realtime_transcription only claimed once that assembly is genuinely live |
Known gaps across the wider backend landscape (context for driver authors)
| Tool / capability | Known gaps at spec-writing time |
|---|
buy_number | Absent on Synthflow (UI-only, no API), ElevenLabs (BYO-number only), Dograh (BYO carrier), Phonely (not independently drivable) |
end_call | Absent on Retell and Synthflow (no hangup endpoint); Dograh has no external hangup endpoint; Vapi supports it only via a per-call ephemeral controlUrl, not a stable REST endpoint |
send_sms | The most degraded tool in the whole contract. Only Telnyx, Bland, Autocalls, and Thoughtly expose real standalone SMS. Vapi/Retell/Synthflow are agent-mediated only (not a stable API surface, never claimed as supports_sms: true). Millis, Vogent, and Dograh have no SMS capability at all. AgentLine's own public materials contradict themselves on this (SKILL.md forbids SMS while its API spec defines POST /v1/messages) — cited as unresolved upstream, not resolved on AgentLine's behalf |
get_recording | Absent on AgentLine entirely; LiveKit-based backends require standing up a separate Egress service before it's real |
get_transcript (realtime) | LiveKit/Pipecat-local stacks require DIY event capture; Bolna's transcript endpoint is unconfirmed; Telnyx's exact transcript route is unverified |
A driver claiming a capability it can't demonstrate against the conformance suite fails CI (see SPEC.md §6.2) — this table is a snapshot, not a promise about backends CallMCP doesn't control, and any driver author whose backend's real capability differs from this snapshot updates their own manifest, not this README.
Repo shape
callmcp/callmcp
├── SPEC.md canonical tool contract (start here)
├── packages/
│ ├── server/ @callmcp/server — MCP core: transports, driver
│ │ registry, config resolution, approval state
│ │ machine, elicitation + fallback-URL flow,
│ │ dynamic tools/list
│ ├── driver-interface/ @callmcp/driver-interface — TS interface,
│ │ capability manifest types, conformance test harness
│ ├── driver-kaicalls/ hosted default driver
│ ├── driver-dograh/ local/self-hosted driver
│ └── driver-byok/ bring-your-own transport + bring-your-own LLM
├── examples/ Claude Desktop / Claude Code config blocks per leg
├── server.json official MCP Registry manifest — published as
│ ai.callmcp/server (registry.modelcontextprotocol.io)
├── smithery.yaml Smithery container deployment config
└── Dockerfile self-host / Smithery container build
Packages
Contributing a driver
Implement the interface in @callmcp/driver-interface, ship a callmcp.manifest.json per SPEC.md §6.1, and run the conformance suite. A capability your manifest doesn't claim true simply results in that tool being absent from tools/list — that's the whole mechanism, not a workaround.
License
MIT for this repository's code. SPEC.md is released under Apache-2.0 as documented in its own header, specifically so anyone can implement a conformant driver without asking permission.