Send signal to smartphone via bluetooth
Using a Bluetooth connection would be an efficient way to do this for that distance.  The added fact that many smartphones already have this interface built in is another plus.  (Yet another answer would be to just use another smartphone and send a text message.)  Zigbee is yet another wireless standard, but I don't think there are many smartphone that have this as yet

Here is a very similar question with some feedback that was asked a while ago:
A Bluetooth device that will send data to a mobile phone

On several web sites you can easily purchase small low cost microcontroller boards (Arduino like) and also Bluetooth transceiver boards.  Here is one source of Bluetooth and Microcontroller boards:
http://www.adafruit.com/products/1697
http://www.adafruit.com/category/261  

These days even Amazon sells boards like this.