The concept is easy: if a prospect calls a trackable phone number, you (the advertiser) pay a fee. So, instead of directing people to call the Telecharge/Ticketmaster phone number, you would direct them to call a trackable phone number in your advertising on Google.
The caller is then instantly redirected to Telecharge/Ticketmaster. The caller never knows that they were redirected.
In exchange, you simply pays a fee to Google for them generating the lead/phone call.
It's simply "pay-for-performance" marketing which has proven itself time and time again in the online world.