> ## Documentation Index > Fetch the complete documentation index at: https://docs.lobbystack.com/llms.txt > Use this file to discover all available pages before exploring further. # Get the extra phone numbers add-on > Asks Polar for a page where the customer pays for the extra phone numbers add-on, which bills extra numbers every month apart from the plan, on an annual plan too. Poll GET /billing/extra-numbers/checkout/{checkout_id} until status is ready, send them checkout_url, then poll until add_on_active is true. Only a business that can buy a priced extra number and doesn't have the add-on gets one; an active add-on, a business that can't buy extra numbers, someone else's open checkout and a deployment that doesn't sell extra numbers are a 409. With a platform key the checkout is made out to the business's owner. ## OpenAPI ````yaml /api-reference/openapi.json post /billing/extra-numbers/checkout openapi: 3.1.0 info: title: LobbyStack API version: v1 description: >- Read calls, contacts, appointments and messages, book appointments, and subscribe to webhooks. Authenticate with an API key in the Authorization header. A platform key also names the business it acts on in the LobbyStack-Business header. Each key allows 120 requests per minute by default. servers: - url: https://app.lobbystack.com/api/v1 security: - bearerAuth: [] tags: - name: Business - name: Receptionist - name: Agents - name: Billing - name: Phone numbers - name: Onboarding - name: Team - name: Website chat - name: Calendars - name: Notifications - name: Calls - name: Contacts - name: Appointments - name: Messages - name: Knowledge - name: Webhooks - name: Platform - name: Partner paths: /billing/extra-numbers/checkout: post: tags: - Billing summary: Get the extra phone numbers add-on description: >- Asks Polar for a page where the customer pays for the extra phone numbers add-on, which bills extra numbers every month apart from the plan, on an annual plan too. Poll GET /billing/extra-numbers/checkout/{checkout_id} until status is ready, send them checkout_url, then poll until add_on_active is true. Only a business that can buy a priced extra number and doesn't have the add-on gets one; an active add-on, a business that can't buy extra numbers, someone else's open checkout and a deployment that doesn't sell extra numbers are a 409. With a platform key the checkout is made out to the business's owner. operationId: createExtraNumbersCheckout parameters: - name: LobbyStack-Business in: header description: >- The business a platform key acts on. Required with a platform key. A per-business key may leave it out; if it sends one, it must be the key's own business. schema: type: string format: uuid - name: Idempotency-Key in: header description: >- A unique value, up to 255 characters, that makes retries of this request safe for 24 hours. schema: type: string maxLength: 255 requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/ExtraNumbersCheckoutCreate' responses: '202': description: Accepted content: application/json: schema: type: object required: - data properties: data: $ref: '#/components/schemas/ExtraNumbersCheckout' '400': description: >- The request is invalid (invalid_request), or a platform key sent no LobbyStack-Business header (business_required). content: application/json: schema: $ref: '#/components/schemas/Error' '401': description: The API key is missing, invalid, or revoked (unauthorized). content: application/json: schema: $ref: '#/components/schemas/Error' '403': description: >- Refused. insufficient_scope: the key lacks the billing:manage scope. forbidden: a per-business key named another business in LobbyStack-Business. key_owner_inactive: the person the key acts for can't any more: a per-business key's creator is no longer an owner or admin, or a platform key's owner_email has no verified LobbyStack account. content: application/json: schema: $ref: '#/components/schemas/Error' '404': description: A platform key named a business it cannot act on (not_found). content: application/json: schema: $ref: '#/components/schemas/Error' '409': description: The request conflicts with the current state. content: application/json: schema: $ref: '#/components/schemas/Error' '422': description: >- The Idempotency-Key was already used with a different request body (idempotency_key_reused). content: application/json: schema: $ref: '#/components/schemas/Error' '429': description: >- Too many requests (rate_limited). Wait for the Retry-After header's number of seconds. content: application/json: schema: $ref: '#/components/schemas/Error' headers: Retry-After: schema: type: integer description: Seconds to wait. security: - bearerAuth: [] components: schemas: ExtraNumbersCheckoutCreate: type: object properties: agent_id: description: >- The agent whose phone page Polar sends the payer back to. Left out, they land on the billing page. type: string format: uuid additionalProperties: false ExtraNumbersCheckout: type: object properties: id: type: string format: uuid description: Stable identifier (UUID). status: type: string enum: - pending - processing - ready - error description: >- pending or processing: Polar is making the page. ready: checkout_url is set. error: making the page failed; start another checkout. checkout_url: description: >- The Polar page where the customer pays for the extra phone numbers add-on. Send it to them. type: - string - 'null' error: type: - string - 'null' add_on_active: type: boolean description: true once Polar confirmed the payment and the add-on is active. created_at: type: string format: date-time description: ISO 8601 timestamp in UTC. updated_at: type: string format: date-time description: ISO 8601 timestamp in UTC. required: - id - status - checkout_url - error - add_on_active - created_at - updated_at additionalProperties: false description: A Polar checkout for the extra phone numbers add-on. Error: type: object properties: error: type: object properties: code: type: string enum: - invalid_request - unauthorized - forbidden - insufficient_scope - not_found - conflict - idempotency_key_reused - idempotency_request_in_progress - booking_disabled - booking_requires_confirmation - slot_unavailable - rate_limited - rate_limit_unavailable - method_not_allowed - internal_error - business_required - key_owner_inactive - plan_limit_reached - phone_numbers_unavailable message: type: string details: type: array items: type: object properties: path: type: string message: type: string required: - path - message additionalProperties: false required: - code - message additionalProperties: false required: - error additionalProperties: false securitySchemes: bearerAuth: type: http scheme: bearer description: >- An API key from Settings > API keys (lsk_...), which acts on its own business, or a platform key from the PLATFORM_API_KEYS setting (lsp_...), which onboards and manages businesses. Scopes: business:read, business:write, calls:read, contacts:read, contacts:write, appointments:read, appointments:write, messages:read, knowledge:read, knowledge:write, webhooks:manage, billing:manage, phone_numbers:manage, team:manage, businesses:create, users:create, phone_numbers:purchase, partner:read, partner.businesses:write, partner.demos:write, partner.packages:write, partner.branding:write, partner.templates:write, partner.team:write, partner.billing:read, partner.billing:write. Only platform keys can hold businesses:create and users:create, and only a key a partner made in its console can hold the partner scopes, which the Partner endpoints use. Only a partner's owners grant partner.billing:read and partner.billing:write. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.