Calls and messages are a clinical safety point as much as a convenience. The practice decides how calls are answered and triaged, how messages left for practitioners are recorded and passed on, how quickly electronic messages are acknowledged and answered, and how urgent clinical content in any message reaches a clinician. Staff know the limits of what can be dealt with by phone or email. Where the practice offers electronic communication it tells patients the response time to expect and that email is not a secure channel for sensitive information, and it gives patients a way to opt in.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.