My guess is that the problem is with the Android Doze function that saves battery … essentially after a period of time it turns off the WiFi radio. Wake-up methods are provided and implemented by Republic, but it appears that there are cases where the wake up is not effective
Users can see this problem if you leave you phone idle (in a normally good WiFi area) for about an hour … when waking the phone you will see the RW Arc go from OWA (Open White Arc …cell) to SWA (Solid White Arc … WiFi).
Normally in a a Cell/WiFi area you would then receive the call over Cell after Republic determined it couldn’t reach you via WiFi however if in a poor or fringe cell area the call will revert to Voicemail.
Here is a response in the thread that @amitl referenced from on of the Republic staff (the GCM message is the wakeup message)
@seanr Jan 17, 2017 11:54 AM
Our QA team has been following this thread and has done many of the same tests you all have. They did noticed that there are some scenarios where the GCM message is not getting to our app from Android to wake us up. We have a fix for most of those scenarios coming in our next Republic App update. I do not have a date yet but a few weeks sounds about right.