security - Phone number detected by a web page when browsing from chrome for android

I recently got subscribed to some alerts by the mobile provider automatically and some amount deducted from my phone balance. I called up the customer care centre and they agreed to refund the amount, and said that it was activated from the internet, by some site which captured my number while browsing. Considering the customer care guy was right, isn't this a big security issue, if any website can access the phone number of their visitor?