Call Transfers

Configure cold or warm call transfers to fixed destinations or Phone Number contact properties.

Warm transfers are temporarily unavailable for Duo agents. Cold transfers remain available. Saved warm-transfer destinations are skipped during calls.

Call transfers let your AI agent hand a live call to a human. You configure one or more transfer rules per agent, and each rule can use a fixed destination or a Phone Number property from the contact. The AI decides when to use each rule based on your prompt. telli supports two types: cold (instant handoff) and warm (the AI briefs the human first).

---

Cold vs. Warm Transfer

Cold Transfer

The caller is connected directly via SIP REFER. Fast, zero delay, and the only type that supports SIP URI targets and custom SIP headers.

Downside: If the destination doesn’t answer, the caller is stuck hearing ringing and the AI cannot return. No context is passed to the human.

On a custom SIP trunk, your provider must permit SIP REFER on the trunk. Warm transfers work without it.

Warm Transfer

The caller is placed on hold while the AI calls the human in a separate session, a briefing agent summarizes the conversation, and the human confirms before the caller is connected. The human gets full context, and if they decline, don’t answer, or hit voicemail, the AI returns to the caller gracefully.

Downside: Slower (caller on hold), and phone numbers only—no SIP URIs.

When to Pick Which

Scenario	Recommended
Transfer to a call center queue (always staffed)	Cold—fast, and someone will always pick up
Transfer to a specific person who may be busy	Warm—AI returns gracefully if they don’t answer
You need to pass call context to the human	Warm—the briefing agent summarizes everything
PBX/SIP integration with custom routing	Cold—supports SIP URIs and custom headers
Transfer to a voicemail-heavy destination	Warm—detects voicemail and returns to caller

---

Warm Transfer Briefing

The briefing is what the AI tells the human before connecting the caller. In Auto mode (default) the AI summarizes who the caller is, what they called about, what was discussed, and what they need, then asks “Are you ready to take the call?”

You can supply a custom briefing prompt instead. It replaces the default instructions, but the system still appends the full conversation history, instructs the agent to ask permission before connecting, and provides the connect_to_customer, wait_for_supervisor_to_return, voicemail_detected, and supervisor_unavailable tools. Briefing prompts support contact variables like {{contact.name}}.

During the briefing the human can accept (caller connected), ask for a moment (the AI stays silent and checks back after 30 seconds, mentioning that the caller is still holding), say they’re unavailable (AI returns and offers alternatives), or not answer (AI returns once the ring timeout expires: 5–120s when configured, 5 minutes when left empty).

---

DTMF and SIP Headers

Post-dial DTMF—Send touch-tones after the destination answers, for IVR menus or extensions. Supports 0-9, A-D, *, #, and w (0.5s pause). Example: 123w45#. Supports contact variables (1w{{contact.extension}}#). Available for both cold and warm.

Custom SIP headers (cold only)—Attach headers to the REFER request for call correlation, routing, or analytics. telli always includes X-Telli-Call-Id automatically so you can match the transfer back to the original call. Add your own name/value pairs under the “Advanced” section of a cold destination; values support contact variables. Not available for warm transfers.

---

Limited Hours

Each destination can have its own optional schedule: enable/disable per day, time windows per day, and a timezone (e.g. Europe/Berlin). You can also block specific dates (holidays), inheriting from account-level no-call dates or disabling per destination.

When someone requests a transfer outside the configured hours:

• Auto (recommended)—The AI informs the caller transfers aren’t available, optionally including the schedule and next available slot. You can toggle the schedule details off.
• Manual message—The AI speaks an exact message you provide, verbatim, with no improvisation.

---

Routing Logic in the Prompt

The transfer tool gives the AI the ability to transfer; your prompt tells it when. A typical multi-destination pattern:

## Transfer Routing

- Pricing, quotes, or purchase questions → @transferCall:sales
- Technical issues, bugs, or error messages → @transferCall:support
- Billing disputes or invoice questions → @transferCall:billing
- Explicit request for a manager → @transferCall:manager

## When NOT to Transfer
- Do not transfer if you can answer the question yourself
- Only transfer for complex issues you cannot resolve
- Always confirm with the caller before transferring

Common rules also include ending the call when the question is answered, using @scheduleCallback when the destination is unavailable, and summarizing what you’ve tried before transferring as a last resort.

Route each contact to a different destination

Use a contact property when each contact needs a different transfer destination.

1. Create the destination property

   Create a contact property with the Phone Number type.

2. Set the value before the call

   Set a valid E.164 value with a leading + for each contact. Populate it in the telli app, by CSV import, through the Create Contact or Update Contact API, or from the Contact Lookup Webhook.

3. Select the property

   In the Agent Builder, open Tools → Transfers, select Add transfer tool…, and choose the property in Transfer target.

4. Test the routing

   Place a test call for a contact that has the property, trigger the transfer rule, and confirm that the expected destination rings.

See Call Transfer for runtime behavior and limitations.

---

Troubleshooting

Issue	Cold	Warm
Destination doesn’t answer	Caller hears ringing indefinitely; AI cannot return (SIP limitation)	Cancels after the configured ring timeout (range 5–120s, empty means 5 minutes); AI returns to caller
Voicemail on destination	No detection; caller connected to voicemail	Briefing agent detects within 10–30s and returns to caller

Recommendation: If your destination might not answer or go to voicemail, always use warm transfer and set a reasonable max ringing duration (30–60s recommended).