This solution uses a Raspberry Pi with Raspbian and a Servo. The servo is connected directly to the pi, and the pi gets its power from a power outlet (Used a mobile powerbank as a temporary solution).
The pi is connected via wifi to my local network with a static ip (kept it local for security reasons).
Step 1: Node.js Webservice
In order to command the servo I used Node.js as a webservice and wrote a simple script that executes shell commands which tell the servo to turn and "click" the elevator button.
Step 2: IOS App
To make things truly "Dumb & Lazy" I wrote a simple iOS app, that has a big button and a state indicator, when the state is green the app is connected to the ip server and your able to click the elevator button.
Great for having friends over no need to call them up every time someone arrives (the elevator has a lock which people without a key to use it).
If people show interest I will write a simple tutorial.