The X-Msw-Companion-Device header is a custom SIP header used by Sentinel VoLTE when a companion device may be used on a call.

The terminating MMTel-AS instance inserts this header in the initial INVITE if the served user’s Metaswitch-TAS-Services transparent data indicates that they have a companion device. The header conveys this information to the terminating SCC-AS instance. The T-ADS features in the SCC-AS will use this information to decide whether the INVITE should be forked to the companion device.

  • The header is for internal use only and is automatically removed when the INVITE exits the terminating SCC-AS instance.

  • The header will also be removed at the MMTel-AS if retargeting has occurred.

Header Format

The header name is X-Msw-Companion-Device, and its value is the radio access type supported by the companion device.

Currently the only supported value is PS, indicating the subscriber has a companion device supporting packet-switched radio access types, e.g. LTE or WiFi.


Companion device support is an experimental feature, so the header format may change in a future release.


X-Msw-Companion-Device: PS
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Sentinel VoLTE Version 3.0.0