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I had a look at your project because I have created a similar system, I just wanted to say that the power usage is excessive, even for Wifi. I have managed to create battery powered (4 AA Batteries x 1600 mAh) Arduino temperature sensors that communicate with a central node (Raspberry Pi) via NRF24L01 radio modules (that you can purchase for 2€ from China), this all amounts to a lower cost per sensor and a much better battery life (more than 6 months). I have documented some of my work here: https://github.com/liamjack/Arduino-NRF24L01-Thermometer
The main node receives all of the temperature data which it can then store locally or send off to a server via Wifi (with a Wifi dongle), Ethernet, GPRS / 3G / 4G (with a GPRS / 3G / 4G dongle)... The server then handles sending alerts (emails, sms via Plivo), no need to hook up an old phone to anything, the SMS sending is done via the Plivo API (7 cents / sms)
I just find it crazy that buying a 400$ battery is even listed as a "solution" to the problem.