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.
If my response to you can't wait for my next normal working hour, then that means I will have to carry my work communication device with me when I leave work and I will not be able to travel anywhere that puts me away from whatever communication type that you require for longer than the duration that you are willing to wait. This type of restriction has a term for it, it is referred to as being "on-call". Employees deserve to be compensated for being on-call. If your business requires your employees to be on-call, but you don't compensate them, then I have very little sympathy for you when you get sued or fined.
I think there is a difference between being on call and being expected to text your boss sometimes. You could literally just check your phone once a day at the same time and it would be fine. Maybe they should throw you a dime for that few seconds of grueling labor, but I don't really give a shit.
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.