WebRTC softphone provisioning 422 on AU-1 BYOC

What’s the best way to enable WebRTC softphone for users via Terraform on AU-1 BYOC?

provider genesyscloud 2.5.1
terraform 1.6.0
genesyscloud_user_settings

Apply fails with 422 Unprocessable Entity. API trace shows validation error on webrtc_enabled field. Plan succeeds.