Hey Tareq,

I understand that you don't hear tone when activating Siri. That is something that has change in iOS 9 and your iPhone now vibrates when it is activated. If you have headphones on, you will hear a chime. For more information on that, take a look at the article below. 

Use Siri on your iPhone, iPad, or iPod touch
https://support.apple.com/en-is/HT204389
 

If Siri doesn't make a sound when you hold down the Home button

On your iPad or iPod touch, Siri will chime when it’s time to ask a question. If you have an iPhone with iOS 9 or later, Siri will vibrate instead. You'll only hear the chime on your iPhone with iOS 9 or later when you're using headphones.

I know you said that you don't hear anything afterwards either with her response. If that is the case, look over the troubleshooting article below for issues with the speaker on your iPhone. 

If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
https://support.apple.com/en-us/HT203794

Take care