Call will go straight to voicemail if phone is off, or more likely in your case if the system cannot see (find) the phone at the time of the incoming call. A text will be stored on the host computer until it can be delivered but the call is realtime so voicemail is the only option. Start checking signal strength if there are particular areas/environments where the problem is more prevalent.
The kicker will be if the voicemail is delivered shortly after you move to an area of strong signal.
Yeah, could be it. Signal really sucks at my house. Dropped calls all the time.