Welcome, Sunshine_777, to Apple Support Communities!
I’m not a lawyer, but it looks like Apple is trying to avoid legal entanglements with Law dealing with “protected health information” (PHI).
Considering the variety of PHI laws, with their highly varied breadths, I can certainly understand Apple trying to avoid such legal entanglements.
It has nothing to do with how secure iCloud is, but the legal entanglements that Apple, or any of its subsidiaries, would be subject to, involving potentially highly intrusive PHI Laws.
As for you question:
«How do doctors use iPhones to receive patient texts, voicemails, etc. if the Terms And Conditions require the doctor to NOT use the cloud for PHI?»
First, emails: use email services other than iCloud email.
This, then, is subject to the conditions of the email service being used.
For the sake of PHI, the doctors would, likely, be best served by using an email service provided as a part of their practice/organization. (Such will not be backed up on iCloud.)
Second, texts, voicemails, etc.: Such messages would, likely, not be directed to the doctors personal phone, and, perhaps, not even to the doctors assigned, organization’s phone, but an organization’s messaging system, similarly to emails, for access by the appropriate staff.
This, then, makes the doctors, and their associated organizations, appropriately responsible under the PHI laws under which they are governed.
Additionally, their organizations are the best equipped to deal with their local PHI laws.