Short answer: If you are building voice calls into your own product, use a voice agent platform with an API: Vapi, Retell AI and Bland all let you start a call with one request and publish per-minute prices. If you only want your own AI agent to make the occasional real call for a person, you may not need a voice stack at all; an MCP calling connector such as OsmO adds a calling tool to the agent instead.
Vapi, Retell and Bland sell minutes, numbers and an outbound call API; you design the agent.
Published starting prices range from about $0.05 to $0.14 a minute, plus extras such as telephony.
OsmO has no REST API. It is an MCP server at api.getosmo.app/mcp with OAuth 2.1 and PKCE.
Pick a platform for volume and control; pick MCP when an agent needs to call on a user's behalf.
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
Two different problems
Build a voice agent when calls are your product. Add a calling tool when calls are an errand.
OsmO"AI phone call API" covers two jobs that are easy to confuse. The first is building a voice agent: you own the prompt, the voice, the numbers and the call flow, and you run many calls for your own customers. The second is giving an AI agent a phone: an assistant such as Claude Code or a custom agent needs to ring a business now and then, for a specific person, and report back. Platforms solve the first. A calling connector solves the second.
Voice agent platforms compared
| Vapi | Retell AI | Bland | |
|---|---|---|---|
| Starting price | $0.05/min hosting, models at cost | $0.07–$0.31/min pay as you go | $0.14/min on the free Start plan |
| Free to try | $5 in credits | $10 in credits | No card on Start |
| Outbound call | /call endpoint or SDK | create-phone-call (v2) | POST /v1/calls with a task |
| Numbers | Included on plans; import for outbound | Managed or imported | Built-in Twilio or your own |
| Also notes | Hosted MCP server | Phone-menu navigation, transfers | Pathways, knowledge bases |
All three publish more than this: concurrency limits, telephony charges and enterprise tiers. Telephony is often billed separately, so price a full call, not just the headline rate. Vapi's docs say outbound calls need a number imported from a carrier such as Twilio or Telnyx.
Prices and features on these pages change often, and each vendor counts minutes, concurrency and extras in its own way. Treat the table as a starting point for your own test, not a verdict: run the same short call on each and compare the bill and the transcript.
Vapi
Vapi describes itself as a platform for building voice agents with a full model catalogue and testing tools. An outbound call takes an assistant, a phone number and a customer number, and can be scheduled or batched. It also runs a hosted MCP server, so an AI client signed in to your Vapi account can create calls and manage assistants; you still build those assistants and own the numbers.
Retell AI
Retell lists call transfer, appointment booking, knowledge-base answers, phone-menu navigation and batch calling, with webhooks and an API. Its create-phone-call endpoint takes a from and a to number in international format.
Bland
Bland's per-minute price includes the language model, speech-to-text and text-to-speech. A call can be started with a phone number and a plain-language task, and higher plans add live transfers and scheduling.
What you still build on a platform
A voice platform gives you the parts, not the finished agent. You still write the prompt and test it against real calls, decide what happens on voicemail, menus and transfers, store the results, and watch costs as volume grows. You also own the compliance side: consent for the calls you place, disclosure, and recording rules where you operate. Budget time for all of it, not just the first demo call.
- How is a full minute priced once telephony, the model and the voice are added?
- How many calls can run at once, and what does more concurrency cost?
- Can you bring your own numbers or carrier, and what does outbound need?
- How do you get the transcript and outcome back: webhook, API or both?
- What does the vendor say about phone menus, transfers and voicemail?
Where OsmO fits, and where it does not
OsmO is not a voice-building platform. There is no agent prompt to write and no campaign tool, and it has no REST API. What it offers is a ready-made calling tool for AI agents, over the Model Context Protocol:
- Endpoint
https://api.getosmo.app/mcp, Streamable HTTP. - OAuth 2.1 with PKCE, and both Dynamic Client Registration and Client ID Metadata Documents, so most MCP clients connect without setup.
- The person signs in with their own mobile number and approves access; calls are placed for that person.
- It calls listed businesses and toll-free numbers in the person's own country (supported: US, Canada, UK, Australia), at most 5 calls to one number in any 24 hours.
Choose a platform if you need your own voice, your own numbers, high volume or inbound answering in your product. Choose a connector if your agent just needs to make a real call for the person using it.
Adding it is one line in most agent tools, for example claude mcp add --transport http osmo https://api.getosmo.app/mcp. Full details are on the OsmO MCP page. 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.
Questions people ask
Does OsmO have a phone call API?
Not a REST API. OsmO is reached through its MCP endpoint, which any MCP client that adds a remote server can use after the user signs in.
Which voice platform is cheapest?
Headline rates differ, and telephony, models and concurrency are often extra. Price one full call on each vendor's page for your own setup.
Can I use OsmO for sales or bulk calls?
No. It calls businesses for a signed-in person, at most 5 calls to one number in any 24 hours. Bulk calling belongs on a voice platform.
Sources
Checked November 13, 2026. Vendors change their products often; follow the links for the current version.
- Vapi pricing
- Vapi docs: outbound calling
- Vapi docs: MCP server
- Retell AI pricing
- Retell docs: create phone call
- Bland pricing
- Bland docs: send call
- Claude Code docs: MCP