No, sometimes you just need to tell an employee something, and then have them verify they understand the information. In my opinion that does not qualify as work.
Because everything important happens during every employees working hours, and it is inconceivable that a plan could change for any reason while you aren't clocked in.
If you want work to get done you have to be coordinated. Just text back, its not that fucking hard.
I would agree that your boss shouldn't expect you to answer at all hours of the day or even remotely quickly, but if you literally never answer anything then I have very little sympathy for you when you get fired.
There shouldn't be anything that requires the employee not be informed the next working day. If it's such an emergency you can't wait then you should call your doctor not your employee.