Enable SIP X header support for context transfer

Important

Some of the functionality described in this release plan has not been released. Delivery timelines may change and projected functionality may not be released (see Microsoft policy). Learn more: What's new and planned

Enabled for	Public preview	General availability
Admins, makers, marketers, or analysts, automatically	-	Sep 2025

Business value

SIP X header support extends in-call context transfer beyond the existing 128 character User-to-User (UUI) limit. Organizations can now pass up to five custom SIP X headers per call, each supporting up to 256 characters. This enables more detailed context such as call ID, language preference, or handling instructions to flow between external systems and voice agents (IVR). The result is faster resolutions, reduced repetition, and improved customer experiences.

Feature details

This feature enhances contextual data exchange between voice agents and external systems in Dynamics 365 Contact Center environments using Direct Routing telephony. It is especially useful for voice agent makers and system integrators who require advanced data-sharing capabilities.

Previously, the SIP User-to-User (UUI) field—with its 128-character limit—was the only method available for passing call context. This update introduces SIP X-Header support, enabling more flexible and scalable context transfer.

The feature offers two key capabilities:

• Receiving SIP X-Headers: Voice agents can access incoming SIP X header values by defining variables that match the header names.
• Sending SIP X-Headers: During external phone number transfers, voice agents can select up to five variables to be passed as SIP X headers.

Technical limitations include:

• Maximum of five SIP X headers per call (for both incoming calls and external transfers).
• Header names limited to 64 characters.
• Header value limited to 256 characters.