@Bebe1980 

This is not really something that is done at a network level, its more an admin task if honest.

You would need to send a few messages, one for each type of device you have on there.   For example Windows devices, use NET SEND and compose a message that way.

IOS and Android will be more difficult, you would need to look at APNS and Google Cloud Services to accomplish that.   

Ultimately, its not something that can be done at the network level, its a layer 7 app command.

  @Philbert 

Thanks

Seems "net send" went out with windows [ark] xp :)

The alternate for the likes of Windows 10 appears to be msg.exe 

  @Bebe1980 

LOL..  Showing my age now!

If you have an MDM in place then this would be an option for you, that is how we send our broadcast messages.. rare as it is

  @Philbert 

I'm sure you're younger than me :)

I won't do it often so I'll just do it twice - they imply Google services does it but that doesn't seem to work on my phone so I'll look for an app.

  @d0ugmac1 

Thanks ... yes I could do something like that