I have supported a similar scheme in a secure hostel for troubled teenagers. The hostel had been suffering from a spate of deliberate breakages. However, in this instance there were only five staff and two escape routes in a small establishment, so it has been easy to ensure ongoing compliance. When parents visit the site they are provided with a key, which is signed for on issue and on return for each occasion.
Call points in locked cabinets are also the norm for sports stadia with reliance on trained stewards to raise the alarm.
Where there is a clearly identified problem, and a robust management solution can be assured, then locked call points may be part of the toolkit in a properly risk assessed approach. The difficulty is in being sure of the robust management solution.
Paul