@Geoff Bruce-Payne Below is the response from Product Team, I hope this helps with your query.
Wake-up via IoT Hub is not possible as you will need an additional channel for activating the device (client) to connect via TCP to IoT Hub. SMS are still used, while definitely not a very secure way. Ripple control is also a way used by utilities to activate a more powerful communication channel. Alternatively the device could automatically connect based on a local event or regularly via timer.
When a device is connected, syncing the Device Twin's desired properties is a good way for "standing instructions" or you utilize the "device connect" event (EventGrid) to trigger a Azure Function with a method call. Few customers also use the "device connect" event to en-queue C2D messages due to the missing support of methods or Device Twins in their devices.
Do let us know if you have any further queries.
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