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Works with GPT-4, GPT-5, AssistantsGive GPT a voice
Callsy plugs into any OpenAI agent — GPT-4, GPT-5, Assistants API, or the Responses API. Your agent gets 27 tools to place real phone calls, manage contacts, and run voice workflows.
Listen to the voices GPT will use
50+ voices across 40+ languages — click any to hear a sample.
Why GPT + Callsy
Function calling is how GPT reaches the world.
Callsy is how it reaches the phone
OpenAI gave you function calling so agents can browse, search, and execute. Voice was the last channel that didn't have a clean tool surface — building it yourself means juggling Twilio, Deepgram, ElevenLabs, and an agent runtime. Callsy ships it as one function call.
Two integration paths
Native MCP via mcp.callsy.ai/sse, or OpenAI function tools via REST. Either way, 27 tools, zero bridging code you need to maintain.
Works with every surface
Assistants, GPTs, Responses API, Agents SDK, LangChain, LlamaIndex, CrewAI. Anywhere GPT calls functions, it can call Callsy.
Streaming-friendly
make_call returns a call_id instantly. Your agent keeps responding to the user while the phone call runs in the background.
Sub-processor transparency
We list every downstream provider (ElevenLabs, Twilio, Deepgram, etc.) in the Annex. No hidden AI layers, no surprise routing.
Setup
Path A: function tools · Path B: MCP
Most teams start with function tools. Register Callsy as a tool on your OpenAI client:
// Pass this tool into your OpenAI function-calling setup
const tools = [{
type: "function",
function: {
name: "callsy_make_call",
description: "Place a real phone call via Callsy.",
parameters: {
type: "object",
properties: {
agent_id: { type: "string" },
contact_id: { type: "string" },
context: { type: "object" }
},
required: ["agent_id", "contact_id"]
}
}
}];
// Then forward the call to Callsy's REST API
await fetch("https://api.callsy.ai/v1/calls", {
method: "POST",
headers: { "Authorization": `Bearer ${process.env.CALLSY_KEY}` },
body: JSON.stringify(args)
});Then just prompt GPT naturally:
Call Sarah about her abandoned cart worth $142.
Offer free shipping. Use Emma's warm US English voice.
Report back with the transcript and outcome.Workflows GPT handles end-to-end
One prompt · Every step
Outbound sales
“Take this CSV of 50 leads, score them, call the top 10, and book demos for the qualified ones.”
Cart recovery
“Pull cart_abandoned contacts from the last 48h. Call them with personalized 10%-off offers.”
Customer support callback
“For every support ticket flagged urgent, call the customer within 10 minutes.”
Renewal outreach
“Call customers whose subscription expires this month. Offer the loyalty discount.”
Frequently asked
How do I connect Callsy to OpenAI?
Two paths. (1) If your OpenAI client supports MCP natively, point it at https://mcp.callsy.ai/sse. (2) Otherwise, register Callsy's REST endpoints as OpenAI function tools — we publish a ready-to-use tool schema.
Does this work with GPT-4, GPT-5, and Assistants?
Yes. Any OpenAI model with function calling can call Callsy. That covers GPT-4 (all variants), GPT-5, Assistants API, and the Responses API.
Can I use this with Agents SDK?
Yes. Callsy exposes the same 27 tools whether your agent framework is the raw OpenAI Assistants API, the Agents SDK, LangChain, LlamaIndex, or CrewAI.
What's the latency?
Tool-call round-trips are typically 200–800ms. The actual call happens asynchronously — make_call returns a call_id immediately, then your agent polls get_call for the outcome.
Will my conversation data train OpenAI?
That depends on your OpenAI account's data settings, not Callsy. Separately, Callsy itself does not train foundation models on your call data.