I’m actually thinking of making phone calls using SIP calling. I have tried achieving this using Tasker with MQTT Client. It can trigger a normal phone call using MQTT message but it couldn’t make phone call using SIP calling. I don’t want normal phone call because the spare phone I plan to use for my main door doesn’t have any SIM card and I don’t plan to get one just for this purpose. Furthermore, SIP calling is better because it can calls to every phone extensions I have in my home including those softphones I have installed in my PC and dedicated SIP phones in every rooms.
UPDATE: I managed to solve this with my old Galaxy Tab 7.7. It has the option to use Internet calling for all calls. You can see my setup at Answer my door anywhere with SIP call using old android phone/tablet, Bluetooth speaker, Tasker and MQTT