Short answer: A phone call MCP server is a connector that gives an AI app a tool for placing real phone calls. The app, such as Claude, ChatGPT, Cursor or Codex, decides when a call is needed and passes the number and the reason; the server makes the call and returns the result. OsmO runs one at https://api.getosmo.app/mcp, with OAuth sign-in by mobile number, so it works in any MCP client that adds a remote server.
MCP is an open standard for connecting AI apps to outside tools; a calling server is one such tool.
The user signs in once with OAuth; after that, the AI can request calls in the chat.
Claude, ChatGPT, Cursor, Codex, Grok and Perplexity all accept remote MCP servers.
OsmO supports OAuth 2.1 with PKCE, Dynamic Client Registration and Client ID Metadata Documents.
How it works
- Sign in with your mobile numberYou get 50 free credits. Nothing to pay.
- Add OsmO to your AI appChatGPT, Claude, Grok and other apps that take connectors.
- Ask it to callOsmO calls, talks, and tells you what was said.
The full guide
What MCP is, briefly
The Model Context Protocol describes itself as an open-source standard for connecting AI applications to external systems. A server offers tools, and a client, the AI app, can call them. Instead of every AI app building its own phone integration, one calling server can work in all of them.
Think of MCP like a USB-C port for AI applications.
Model Context Protocol docsWhat a calling tool looks like
From the AI's point of view, a calling tool is a function with a description. OsmO's main tool is place_call: it takes the reason for the call and who to call, and returns a call id and status the AI can check later. In ChatGPT, Claude, Grok and other connector apps OsmO also offers tools that look up a listed business's number, so the AI does not need you to find it.
- You ask in the chat: "Call the garage and ask if my car is ready."
- The AI looks up the number if needed and confirms with you who it will call and why.
- It calls
place_callwith the number and the reason. - OsmO places the call from the OsmO number for your country, works any menu and waits on hold.
- The AI checks the call and tells you what the business said.
How each app adds a calling server
| App | Where to add it |
|---|---|
| Claude | Customize → Connectors → + Add → Add custom connector (Team and Enterprise: Organization settings) |
| Claude Code | claude mcp add --transport http osmo https://api.getosmo.app/mcp |
| ChatGPT | OsmO in the plugin directory; custom MCP servers on Business and Enterprise |
| Codex | codex mcp add osmo --url https://api.getosmo.app/mcp, then codex mcp login osmo |
| Cursor | A url entry under mcpServers in .cursor/mcp.json |
| Perplexity | Add custom → Add MCP connector → Remote (Pro, Max and Enterprise) |
Grok also takes custom connectors at grok.com/connectors. Our setup pages for Claude, Cursor and Codex walk through each one.
The exact menu names change as vendors update their apps, and some features depend on your plan or on an admin allowing custom connectors. If you cannot find the option, check the vendor's own help page in the sources below, or ask your workspace admin whether custom connectors are switched on.
Sign-in and safety
A calling tool can spend money and reach real people, so how it signs in matters. OsmO uses OAuth 2.1 with PKCE: the person signs in on OsmO's own page with their mobile number and a texted code, then approves the app. Clients that support Dynamic Client Registration or Client ID Metadata Documents connect without any keys to copy. Access can be removed from the OsmO app at any time.
- The AI app should confirm who it is calling and why before every call.
- From an AI app, OsmO calls listed businesses and toll-free numbers, plus your saved contacts if you use the OsmO app.
- There is a cap of 5 calls to one number in any 24 hours.
- In ChatGPT, Claude, Grok and other connector apps, up to 5 numbers can go in one request, for example to compare three shops.
Asking for calls that work
The AI turns your words into the tool's inputs, so a clear request makes a clear call. Name the business and the area, say what you want to know or arrange, and add the details a receptionist will ask for. For a comparison, list the places in one message and the AI can send up to five together. If you want the call later, say so: on a web account, scheduling a call for later is done in the OsmO app.
Calls take a few minutes. Most AI apps will tell you the call has started and come back with the result when you ask, or when it finishes. A call nobody answers costs nothing; a later attempt uses 3 credits if it is answered.
Other calling servers
Developers who already build on a voice platform can manage it over MCP too. Vapi's hosted MCP server lets a signed-in AI client create calls and manage assistants on a Vapi account. That suits teams with their own voice agents; OsmO suits an agent that just needs to make a call for the person using it.
Building your own MCP client? OsmO publishes its OAuth discovery documents under https://api.getosmo.app/.well-known/, and the scopes and error codes are listed on the MCP page.
Try it
OsmO For my AI starts with 50 free credits when you sign in with your mobile number, with no card. An answered call uses 3 credits however long it lasts, and a call nobody answers is free, so the free credits cover about 16 answered calls. When they run out you can keep calling from the OsmO app. Full technical details are on the OsmO MCP page.
Questions people ask
Is there an MCP server for phone calls?
Yes. OsmO runs one at https://api.getosmo.app/mcp for calling businesses, for people in the US, Canada, the UK and Australia. Voice platforms such as Vapi also run MCP servers for their own accounts.
Does OsmO need an API key?
No. It uses OAuth: the person signs in with their mobile number and approves the app.
Which transport does OsmO use?
Streamable HTTP, at https://api.getosmo.app/mcp.
Sources
Checked November 13, 2026. Vendors change their products often; follow the links for the current version.
- Model Context Protocol: introduction
- Claude Help: custom connectors using remote MCP
- Claude Code docs: MCP
- OpenAI Help: custom MCP servers in ChatGPT
- OpenAI docs: MCP in Codex and the ChatGPT desktop app
- Cursor docs: MCP
- Perplexity Help: custom remote connectors
- Vapi docs: MCP server