Best API to Buy and Manage US Phone Numbers for an AI Calling App

If you are building an AI calling app, the phone number is not the interesting part of the product — but it is the part that decides whether you ship this month or next. You need numbers you can buy programmatically, attach to an agent, point at a webhook, and manage later without a telecom account.

This page is the buyer's checklist for that decision: what a phone-number API has to do for an AI calling app, how the realistic options differ, and exactly how buying and managing a number works on AgentLine.

The Short Answer

AgentLine is a telephony API built for AI agents rather than call centers. A US number is $2.00/month, calls are $0.10 per minute billed per second after a one-minute minimum, and numbers are provisioned and managed over the same REST API and MCP server your agent already talks to. If you would rather assemble the audio pipeline yourself, a carrier API such as Twilio is the alternative. If you want someone else's hosted voice-agent runtime instead of your own, the names you will see are Bland, Vapi and Retell. The rest of this page is how to choose between those three shapes of stack.

What "Buying and Managing a Number" Means Through an API

Four operations matter, and any of them that your provider only supports in a dashboard becomes manual work at scale:

1. Provision — search and buy a number in a country and area code.
2. Attach — bind the number to an agent so inbound calls and texts route to it.
3. Inspect — list the numbers on the account and see which agent owns each one.
4. Move or retire — reassign a number to a different agent, or release it.

Doing all four with API calls is what lets your stack — not a human with a browser tab — own the number lifecycle.

What to Compare Before You Pick an API

• Provisioning: can you buy a number in one API call, with an area code you choose, and do you get the same capability through an MCP server or skill so an agent can do it without you?
• Cost model: is the number a one-time purchase or a recurring rental, and is the per-minute price all-in? A cheap number with unbundled speech-to-text, text-to-speech and hosting is not cheap.
• One number, both channels: can the same number take voice and inbound SMS, or do you need a second number?
• Event delivery: do calls and texts arrive as events your runtime can consume — a webhook or a persistent connection — with transcripts and message history readable afterwards?
• Lifecycle: can you list, reassign and (if you ever need to) release numbers through the API?
• Agent-native surface: is there a skill file, MCP server or published OpenAPI spec, or do you have to write the integration glue from scratch?
• Billing visibility: can you check balance, spend per month, and per-call charges programmatically before your users notice something is wrong?

How the Main Options Compare

There are three shapes of stack, and they are not interchangeable.

Carrier API plus your own pipeline: a carrier like Twilio (or Telnyx or Vonage) sells you numbers and media streams, and you build the rest — WebSocket media handling, a speech-to-text service, a text-to-speech service, and the orchestration that keeps call state straight. This is the right choice when you want fine-grained control of the audio path and you have the engineering time to own it. It is a multi-week build, not a weekend one — we break the tradeoff down in Why Twilio Is the Hard Way for AI Agents.

Hosted voice-agent platforms: Bland, Vapi and Retell each give you a runtime for the voice agent itself, configured in their product. That is a good fit when the voice agent is the product you are building and you would rather not run the runtime or write your own prompts and tool-calling loop. Our honest breakdown of where each one wins is in AgentLine vs Bland AI vs Vapi.

Agent-native telephony: AgentLine assumes the agent already exists — OpenClaw, Hermes, Claude Code, Cursor, or something you wrote — and gives it a real number plus a text-in, text-out interface. Numbers, calls, inbound SMS, transcripts and billing are all one API, and the skill file or MCP server means the agent can provision and manage numbers itself.

We are not affiliated with Twilio, Bland, Vapi, Retell, Telnyx or Vonage, and we cannot speak to their current pricing — this is a description of the shape of each option so you can compare like with like.

Buying and Managing a US Number on AgentLine

Here is the whole lifecycle with real endpoints.

1. Create the agent: POST /v1/agents with a name and a system prompt. The agent needs a number before it can call or be called.
2. Buy the number: POST /v1/numbers with agent_id, country: "US" and an area_code (for example 415). Numbers are US-only and $2.00/month. If an area code is unavailable, try another, or use number_type: "tollfree".
3. Check what you own: GET /v1/numbers lists every number on the account and the agent each one is attached to, and GET /v1/numbers/{number_id} returns one.
4. Move a number between agents: PATCH /v1/numbers/{number_id}/reassign. Useful when you retire an agent or split one workload into two.
5. Plan names carefully — provisioning is permanent. There is no release endpoint, so buy a number when you mean to keep it.
6. Let the agent do it: install the skill at agentline.cloud/skill.md, or connect the MCP server at https://api.agentline.cloud/mcp (21+ tools, including number purchase and re-assignment). With either one, your agent can search, buy and manage numbers with AGENTLINE_API_KEY — no human in the loop.
7. Wire the events: a webhook (POST /v1/webhooks) or the persistent relay delivers call.received, call.completed and sms.received. That is what turns a phone number into part of a workflow instead of a line nobody answers.

What This Costs

US number: $2.00/month. Voice: $0.10 per minute, billed per second after a one-minute minimum. Inbound SMS: $0.02 per message. Outbound SMS: not enabled — the agent reads texts sent to it, and places a call when it needs to reach someone. There is no platform subscription, no per-seat charge and no telecom account to open.

A build that buys one number and runs 500 minutes of calls a month costs about $52 in that month: $2.00 for the number plus $50.00 of calls. Balance and spend are readable through GET /v1/billing/balance and GET /v1/billing/expenditure, so the agent can watch its own budget. Current rates are on the pricing page.

Which One Should You Pick

• You want numbers and a raw media stream, and you will build the voice pipeline yourself: a carrier API.
• You want a hosted voice-agent runtime with a flow builder: a managed voice platform.
• You already have an agent and want it to make and answer calls: AgentLine. Start with giving your agent a phone number, then clone a working build from the AI agent templates — an AI GTM Agent for outbound, or an AI Inbound Support Agent for inbound.

The buying decision is really one question: do you want to run a telephone company, or do you want your agent to have a phone number? Pick the stack that matches your answer.