I ended up using automated flow apps (Automate
[link below] is one such app that I used. Disclaimer: I've no affiliation with app) to trigger automated calls. The flow was initiate on a GCM notification to the phone, which provided the number for call. With all permissions granted and conditions fulfilling (network, sim, call credits, etc.), app should be able to make call to the number.
Now, I'm trying to figure out how to play an audio message for the receiver.
So, if you were looking for 'how' part, here's the how part. If you know the next steps, enlighten me!
https://play.google.com/store/apps/details?id=com.llamalab.automate&hl=en&gl=US