BuildActionsCall Transfers

SIP Transfers

Transfer calls to SIP-compatible endpoints.

SIP transfers enable your AI agent to hand off calls to SIP endpoints such as PBX systems, SIP phones, or softphones.

Looking for other transfer types? See the Call Transfers overview for phone number (TEL) and dynamic transfer options.

Prerequisites

  • Your SIP endpoint must be reachable from Synthflow
  • SIP (UDP/TCP 5060) and RTP ports open and forwarded

Setup

1

Go to Agents → [Your Agent] → Actions.

2

Under During the call, click Add Action → Call Transfer.

3

Enter your SIP URI (e.g., sip:user@domain.com).

4

Configure transfer settings:

  • Transfer mode: Warm (with message or context summary) or blind
  • Timeout: How long to wait for the endpoint to answer
5

Click Add Action to save.

Custom X-Headers

You can define custom X-headers in the UI to pass additional metadata with your SIP INVITE requests. This is useful for routing logic, caller identification, or integration with your PBX system.

Custom X-Headers configuration

Header values can be populated using variables or info extractors.

Variables

Use {} syntax to inject dynamic values into a header:

Variable SourceExampleDescription
Pre-call webhook{results.data.name}Data returned from your pre-call webhook
During-call actions{results.data.data.price}Results from custom actions executed during the call
Flow designer variables{variable_name}Variables defined and collected in the flow designer
System variables{call_id}, {from_phone_number}, {to_phone_number}Built-in system variables for the current call

If you use a custom action before the transfer, the transfer waits for the action to resolve before initiating. This ensures variables returned by the action are available in your SIP headers.

Info Extractors

An info extractor lets the AI agent pull a specific piece of information from the conversation and pass it as a header value. Each extractor has two fields:

FieldDescription
HeaderThe X-header name the extracted value is sent in (e.g., X-Customer-Intent)
PromptInstructions telling the agent what to extract (e.g., “What is the caller’s intent? Return one of: make a claim, cancel policy, general inquiry”)

The agent evaluates the prompt against the conversation at transfer time and writes the result into the specified header. Use info extractors when the value you need doesn’t exist as a variable — for example, a caller’s intent, sentiment, or a summary of the conversation so far.

For more on how extractors work, see Information Extractors.

Example: Passing Call Context for Agent Handoff

A common pattern is to combine variables and info extractors so the human agent receives full context on transfer:

Header NameSourceValue / Prompt
X-Customer-IntentInfo extractor”Classify the caller’s intent as one of: make a claim, cancel policy, billing question, general inquiry”
X-Caller-NameInfo extractor”What is the caller’s name?”

Field Requirements

FieldRequirements
NameMust start with X- and contain only letters (A-Z, a-z), digits (0-9), hyphen (-), and underscore (_). Pattern: ^X-[A-Za-z0-9\-_]+$
ValueCannot contain CR (\r), LF (\n), or NULL (\0) characters. Supports variables (e.g., {customer_id}).

Avoid adding too many X-headers. Excessive headers increase the size of SIP INVITE packets, which can lead to packet fragmentation and loss—especially over UDP. Keep your headers minimal and only include data that your PBX or routing logic actually needs.

Supported SIP URI Formats

FormatExampleUse Case
Basicsip:user@domain.comStandard SIP endpoint
With portsip:user@domain.com:5060Non-standard port
Secure (TLS)sips:user@domain.comEncrypted connections
E.164sip:+1234567890@provider.comPSTN gateway routing
Rawinternal-ext-123Custom telephony only (see below)

SIP Methods: REFER vs INVITE

The SIP method used depends on your transfer mode:

Transfer ModeSIP MethodBehavior
Blind (Cold)SIP REFERSynthflow hands off the call entirely; your SIP server handles the new connection
WarmSIP INVITESynthflow creates a new outbound call leg, bridges the calls after whisper/summary

SIP REFER considerations:

  • Caller ID shows the original caller’s number
  • Requires your SIP server to support REFER
  • Transfer happens entirely on your telephony infrastructure

SIP INVITE considerations:

  • Caller ID shows your Synthflow agent’s number
  • Allows whisper messages and conversation summaries
  • Synthflow maintains control until transfer completes

Security

Synthflow supports Access Control Lists (ACL) to secure inbound SIP calls. Configure your SIP server to accept connections only from Synthflow’s IP addresses. Username/password authentication is not currently supported for inbound call transfers.

Transfer Failure Handling

Failure ScenarioBlind TransferWarm Transfer
SIP endpoint unreachableCall endsAgent can retry or continue
Timeout (no answer)Call endsAgent can retry or continue
SIP error response (4xx/5xx)Call endsAgent notified with error

For mission-critical transfers, use warm transfer mode to enable graceful failure recovery and alternative routing.