SIP 500 internal server error "from or target user pool or deployment assignment is incompatible with split-domain traffic type"

Problem: User could not transfer a phone call.

Symptom: Bogus error message about split-domain traffic, with almost no articles on the internet or forums to help. Equally bogus error message was "request target is not assigned to a pool or deployment and is not a server GRUU"

Solution: Disable SIP Refer on the SFB Trunk

Explanation: Not all SBC gateways support SIP Refer, but this is the default option when creating a trunk in Skype for Business.