How can you organize a user notification system?

Question

Notifications

Hello.
Help, please, deal with the problem.
Given:

1. Workflow divided into stages. Each stage has its own start and end time.
   
2. Employees who participate in these stages.
   
3. These employees have smartphones.
   

Need to:
Create an employee notification system. At a certain date and time, the employee receives a call on his work phone. Receives a call, the recording from the bot turns on: so and so, you need to complete the task, the deadline expires in two hours.
Perhaps it will be by means of cellular communications. Perhaps it will be Telegram, Viber or WhatsApp. Perhaps...
It must be a call. A signal that sounds or vibrates in a person until he picks up the phone and either accepts the call or hangs it up. Thus, a one-time message from any bot will not help here.
Please share your personal experience. How did you set up such a notification system.

Answer 1

Thus, a one-time message from any bot will not help here.

Will help. That is exactly how it is done.

1. If unavailable or not answered within 30-60 seconds, the task is postponed for 30 minutes.
   
2. Second time in a row for 2 hours
   
3. Third time at 8 o'clock.
   

Answer 2

You can use the task scheduler built into the OS (crown or Windows, as you like).
For a call, you can take a SIP call service or install your own asterisk.
For messengers, there are a lot of recipes for writing a bot.