What system to use for notification of technical work?

Question

Notifications

The bottom line:
There are a ton of customers, There is a cloud.
Sometimes there is work being done on the network or something similar that can affect the availability of resources. Clients need to be notified of this.
Who uses what?
For now, we use mailing groups in chang, but does anyone know more convenient tools?
Naturally only local. External sites, etc. not interested.