There isn't an actual reg. The FAA issues guidelines only and CASA pretty much follows them. It is up to the airline to determine it's own policy for the use of PED's. The onus is on the airline to prove that operating them is safe or otherwise. That's pretty much why they just said turn them off. A few years ago the FAA (and CASA shortly afterwards) advised that PED use could be permitted as long as they were operated in "Flight Mode". If the airline policy is different, or the use contravenes whatever policy the airline has, then that is where the offence is, by failing to follow a direction of the crew. The FCC, not the FAA, in America ban making mobile calls in flight because of the issues it causes with the terrestrial cell network. The same problem probably exists in Australia, but I'm not sure which agency has the blanket power to prohibit the airborne use of mobile phones for telephony, and I don't think such a prohibition exists.
An excerpt from a position paper from the Australian Mobile Telecommunications Association:
In Australia, the Civil Aviation Safety Authority does not have specific
regulations regarding mobile phone use and has left the decision up to the
discretion of the airlines. The onus is placed on the pilot in command or the
operator to ensure that the aircraft is operated safely. Passengers are required to
obey all lawful instruction issued by the pilot in command in accordance with
Civil Aviation Regulations.
CASA's page:
https://www.casa.gov.au/safety-manag.../mobile-phones