Activate Siri, this is what you need to do

Before you can use Siri, you must first activate Siri. If you have had trouble starting or find Siri in your menu after backing up your apps and data, or you get voice control instead of Siri, you can also activate Siri using the same method as shown below. Also make sure that Siri is turned ON in the settings -> General menu after you have activated Siri.

Here is what you need to do, to activate Siri:

1) Go to Settings -> General -> Restrictions
2) Scroll down and find Siri, you must make sure that Siri and Explicit Language is ON:

3) Go back to Settings -> General, there will now be menu button called Siri:

Make sure that Siri is ON:

Siri is now activated.

This procedure was performed on an iPhone 4S