> ## Documentation Index > Fetch the complete documentation index at: https://docs.poly.ai/llms.txt > Use this file to discover all available pages before exploring further. # SMS link with transfer fallback A common pattern in voice agents: offer to send the caller a link by SMS, but transfer to a live agent if SMS isn't an option or the caller asks for it. The decision lives in a function so it is deterministic — not subject to LLM interpretation. ## Files involved ```text theme={"theme":{"light":"github-light","dark":"github-dark"}} functions/start_function.py ← stash caller_number functions/send_link_or_transfer.py ← SMS or handoff decision config/sms_templates.yaml ← SMS template (alongside other templates) config/handoffs.yaml ← transfer destination (alongside other handoffs) topics/Get Booking Link.yaml ← triggers the pattern ``` ## send\_link\_or\_transfer.py ```python theme={"theme":{"light":"github-light","dark":"github-dark"}} from _gen import * # @func_description( "Called when the caller has been offered the booking link by SMS or by speaking to an agent. " "wants_sms: True if they want the link by text, False if they want to speak to someone." ) @func_parameter("wants_sms", "True if the caller wants the SMS link, False if they want to speak to an agent") def send_link_or_transfer(conv: Conversation, wants_sms: bool): if wants_sms: to_number = conv.state.caller_number if not to_number: return ( "Tell the caller we were unable to send the SMS because we do not have their number. " "Offer to transfer them to an agent instead or ask for a number to send to." ) conv.send_sms_template(to_number=to_number, template="booking_link") return "Tell the caller the link has been sent and ask if there is anything else you can help with." else: conv.call_handoff( destination="agent_queue", reason="caller_requested_agent", utterance="Let me connect you with a member of the team now.", ) ``` ## Get Booking Link.yaml ```yaml theme={"theme":{"light":"github-light","dark":"github-dark"}} enabled: true example_queries: - Can you send me a link to book? - How do I book online? - I want to make a reservation online - Send me a booking link - Can I speak to someone about booking? content: |- Bookings can be made online at book.example.com, or by speaking with a team member. actions: |- Ask the caller if they would like the link sent by text message, or if they would prefer to speak to someone. Use {{fn:send_link_or_transfer}} once they respond. ``` **Testing via `poly chat`: `caller_number` will be empty** `conv.state.caller_number` is populated from inbound caller ID, which is only available on a real voice call. When testing with `poly chat`, `caller_number` is always empty regardless of `--channel` — `wants_sms: True` will always hit the "unable to send" branch. To exercise the SMS path locally, mock the value in `start_function.py`: ```python theme={"theme":{"light":"github-light","dark":"github-dark"}} conv.state.caller_number = "+15551234567" # remove before deploying ``` ## config/handoffs.yaml The `destination` value passed to `conv.call_handoff` must match the `name` of a handoff defined in `config/handoffs.yaml`. All handoffs are defined in a single file under the `handoffs` key. ```yaml theme={"theme":{"light":"github-light","dark":"github-dark"}} handoffs: - name: agent_queue description: Transfer to live agent queue is_default: true sip_config: method: refer phone_number: "+441234567890" ``` See the [handoffs reference](/adk/reference/handoffs) for all SIP method options and field details. ## Per-environment sender number If the sender number differs between environments, the simplest approach is to configure `env_phone_numbers` directly in `config/sms_templates.yaml` — no code required: ```yaml theme={"theme":{"light":"github-light","dark":"github-dark"}} sms_templates: - name: booking_link text: "Here's your booking link: https://book.example.com" env_phone_numbers: sandbox: "+441111111111" pre_release: "+442222222222" live: "+443333333333" ``` The platform selects the right number automatically when `conv.send_sms_template()` is called. See the [SMS setup reference](https://docs.poly.ai/sms/introduction) for full template configuration options. If you need more control — for example, when using `conv.send_sms()` to send free-form content rather than a template — you can read `conv.env` directly: ```python theme={"theme":{"light":"github-light","dark":"github-dark"}} ENV_SENDER_NUMBERS = { "sandbox": "+441111111111", "pre-release": "+442222222222", "live": "+443333333333", } def send_link_or_transfer(conv: Conversation, wants_sms: bool): if wants_sms: from_number = ENV_SENDER_NUMBERS.get(conv.env, ENV_SENDER_NUMBERS["sandbox"]) conv.send_sms( to_number=conv.state.caller_number, from_number=from_number, content="Here's your booking link: https://book.example.com", ) return "Tell the caller the link has been sent." ... ``` **Use secrets for sender numbers in production** Avoid hardcoding phone numbers in function code. Store them as [secrets](https://docs.poly.ai/secrets/introduction) and retrieve with `conv.utils.get_secret("sms_sender_live")`. ## Related pages Structure and variable substitution for SMS templates. Configure transfer destinations used by `conv.call_handoff`. Return values, conv API, and function structure. Full reference for `conv.send_sms_template`, `conv.call_handoff`, `conv.env`, and all other `conv` attributes. Store sensitive values like sender numbers and retrieve them at runtime with `conv.utils.get_secret`. Configuring SMS channels, sender numbers, and template structure.