What is SIP transfer?

Why is SIP transfer important for flexible VoIP routing?

SIP transfer lets an active IP voice session move to another endpoint or service through standardized signaling.

• Routes calls between platforms without rebuilding the media session manually.
• Supports transfers to agents, queues, numbers, or external systems.
• Enables programmable handoffs in cloud voice workflows.
• Provides signaling events for monitoring and recovery.

How SIP transfer works

• The current endpoint or application chooses a transfer target and method.
• A SIP REFER request or a new INVITE initiates the transfer, depending on platform behavior.
• The transferor tracks provisional and final responses while media paths change.
• Success, failure, or timeout logic updates the application state and preserves call correlation.

SIP transfer in action

A voice agent sends a SIP REFER to the contact-center queue. The receiving system accepts, establishes the new dialog, and the application records both call identifiers so the handoff can be traced.

How SIP transfer is measured or evaluated

Evaluate with representative conditions and a published definition. Track these measures together:

• transfer initiation, acceptance, and completion rate
• signaling response and connected-transfer latency
• failure by response code, carrier, target, and method
• media continuity and one-way-audio incidents

Limitations, risks, and common failure modes

• Providers implement REFER, Replaces, and diversion headers differently.
• NAT or SBC policy can block the new dialog.
• Caller identity and custom context may be stripped.
• A signaling success does not guarantee useful media or an answered agent.

SIP transfer vs. warm transfer

SIP transfer describes the signaling mechanism that moves a call. Warm transfer describes the customer experience and coordination with the receiving agent; it can be implemented with several telephony methods.

What teams should consider when using or implementing SIP transfer

• Document the exact transfer method supported by every provider.
• Test blind, attended, internal, and external targets.
• Correlate original and transferred dialog identifiers.
• Define recovery for rejection, timeout, and media failure.