VoIP app (CallKit) not relaying incoming call notifications to paired Apple Watch

Incoming calls reported via reportNewIncomingCall on a CXProvider are correctly presented on the iPhone via CallKit, but are never relayed to the paired Apple Watch. Native cellular calls relay to the Watch correctly on the same devices.

What does a VoIP app's CXProvider need to satisfy for callservicesd to consider it eligible for phone continuity relay to paired Apple Watch?

VoIP app (CallKit) not relaying incoming call notifications to paired Apple Watch
 
 
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