How to build a low-cost GSM door opener

In the last days I once again tinkered a bit. Certainly more bad than good, but where are no professional tools, there won’t be high quality. But theĀ ultimately goal was not a professional product, but rather a simple and cheap solution. But what is it all about?
The product of my tinkering is a low-cost GSM door opener. That is simplified a small device, that opens your door when you call it and may bring the costs next to nothing. And now a little about the implementation …
Anyone who wants to get an overview about the project, can view the following video, which I made from the final product.

To build a similiar application you need the following components:

Nokia 6230i
12V relay (a 5V relay would be better, but I had none)
Connector strips
A few wires

First I disassembled the phone and removed the vibration motor. To the now vacant contacts on the phone board […]