A conceptual question on turning power on/off for a wireless communication
In order for SMS messages to be recieved, there absolutely, definitely has to be power going to the wireless modem. There is absolutely no way around this.

Best thing you can do is to switch the modem on for a short period, say 15 seconds, which should be enough to receive any messages - do this say once every 5 minutes, or once an hour, or whatever. All this can be controlled by the microprocessor, which would have to be running this whole time. (Although it can be in standby mode while doing the timing, which would use virtually no current at all). 

There is no other way. This is why pagers and mobile phones have to be on to receive calls and texts!!! They can't just magically switch on when there's a message or call waiting for them. There has to be active circuitry which does the looking out for a message or a call. And the circuitry which does this, HAS to be the modem. However, rest assured, these GSM modules and so on are designed by very knowledgeable teams following highly optimized standards, and represent state-of-the-art, and are designed to consume as little power as is necessary for this very task you ask of it. So if you use it as it is supposed to be used, (i.e. make best use of its standby features) then you can be sure that you are using the best possible solution.

The fact that you only need to receive a few bytes is inconsequential. The act of receiving an SMS involves far, far more data. And the data isn't even the problem with respect to power: receiving a message involves a complex "Handshake" which involves the thing doing the receiving to actually send data as well. That aspect consumes a lot of power. (And that is on top of the amplifying circuitry that does the receiving). This is why the modem needs to be on to receive messages.

The reason for the handshake being necessary is down to authentication and encryption. The cell tower and network provider need to know that the device in question is the one meant to receive the SMS. So first, when you send an SMS to a device, the network has to first of all find what it thinks is the destination device. So your modem would have to send pings out so the towers know where it is. Then when the tower has an SMS to send, it has to "talk" to the device, in order to authenticate that it is genuinely the correct recipient before sending the data. And all this has to be done with encryption so that firstly, no-one else can intercept the data, and more importantly, so no-one can intercept the authentication credentials (which would allow them to steal your device's identity, and any further data that is sent to it, or fake data coming from it).