Why does it matter which mobile phone I use to activate the parking?

Since your mobile phone is connected to your account, the system will not recognise you if you are not calling from your registered mobile phone. Also, given the fact that you can start a parking session in one way and end it in another the system needs to know which phone is permitted to end the transaction once it started (for example, if you use the "Live Parking" in your online account to start a session but would like to end it by calling the system when you reach your car).