I interpret this as the situation of talking to a single person ‘tu’ about ringing (us) after tickets have been bought (see my note below) for a group of people which includes the listener but not the ‘speakers’. 

 In English we might confirm this by saying a contorted mouthful like  ‘you will ring us when you have (bought) the tickets for all of you’, as we don’t have the luxury of separate words for singular and plural ‘you’ (I am excluding ‘youse’, which is Australian slang !)

Note: the English translation of this sentence seems very literal - at least in my part of the world, we would never use ‘taken tickets’ in this context. Either just ‘have’ or ‘have bought’ or similar is more natural in my view. Given it is at B2 level, these suggested free translations should be understood by most doing the lesson, but a note that it is a free translation rather than literal would clarify.