Therefore as we had an android spare smartphone where we manually forward call to people, I decide to automate all the stuff with Tasker. Since almost one year now, it worked well and now I decided to create a real GUI with same functions ... and more.
I would say that in France, we've got chance to have a mobile carrier challenger named "Free" which sell subscription for 2€/month, which is very cheap. At this point, it's worst and easy to get one and buy a second hand android smartphone. And I'm' pretty sure that such kind of cheap subscription exists around for you.
So DutyManager could really help a lot of people or small businesses to have the cheapest solution for on-call duties management
From what I've tested during last year, the best config for DutyManager is to plug your phone via usb on any "always-on" computer or on a wall power plug; just because the phone need to stay on for calendar synchronization. In my company, our device is in our data room, attached to a server powered up in usb. We've just "plugged and forget". As we have duties every week-end, we know that everything works. However from time to time, device is rebooted manually. Note: if your device is rooted, a reboot remote function is available for that.