problem on what sensor must use on phone detector?

I'm doing my final project and still in confuse on what kind of sensor to use to detect GSM signal. Anybody knows?

A quick google search led me to numerous sensors, some of which are discussed on these forums.

What is the relation to Arduino?

this is what i read. When using with an Arduino board you have to connect the sensor output to an analog input. The measured sensor voltage is equivalent to the received radio signal strength. The 5 Volt to power the sensor can be obtained from the Arduino 5V terminal.

Where did you read that?
So you have a sensor already otherwise you would not have this info?

Take a look on this

http://interface.khm.de/index.php/lab/interfaces-advanced/radio-signal-strength-sensor/

http://cds.linear.com/docs/en/datasheet/5534fc.pdf

i still dont understand a lot about sensor need to use on how to detect gsm signal and how it involves the arduino on this case.