Okay, so I had this idea where I could put a Raspberry Pi into my letterbox with a proximity sensor connected to its GPIO (all battery powered), and it could text me when I get post. 

I have done the research into the functional element (Pi, prox sensor, python to make it all work) but I'm having trouble with the communication element. I was hoping to connect the Pi to the internet, and then it could just use the Twilio library - but our postbox is outside of line of sight, and outside of WiFi range.

Next idea was a mobile broadband dongle, but that would haemorrhage battery power.

My latest idea is to put a crappy (one month standby) phone in the box with it, and try to interface the two so the Pi can actually text me. What I have not found so far is anything that might let me control the phone (or indeed, any compatible phone) - does this kind of thing exist? Or are there any pre-existing projects with similar parameters that anyone knows of?

Thanks for your help!