Hi, I saw the same question someone asked before few months. But, now I am not able to find the same question again.
I guess, we can achieve the result according to one of the app that I created 'Alarm for New Order Recieved'. Taifun made a tutorial for that as 'Alarm (Text to Speech) when New Whatsapp Mesaage'. (Links Given Below).
I think, if we do following three steps we can ring all the phones simultaniously in which our app is installed.
Step 1: First, you have to set up One Signal Push notofication using paid or free extensions.
Step 2: Send some Notification with some specific key word. eg, Apple321
Step 3: Follow Taifun's tutorial as I mentioned above. Design the app in such a way that if the keyword Apple321 is detected in notification then ring Alarm.
I hope, this will work. I will try to edit this post and include the tutorial link.
Links:
Second link is the tutorial link, but you may have to go through both links for better clarity.
Just try your ideas and if you need help, come back and ask here
Btw in case there exists a push notification extension, which is able to set a custom alarm notification sound, then this extension would be everything you need
Taifun
PS: no, I do not know, if one of the push notification extensions is able to do this
