During the Skype interview they ask you why that role, why BT, what do you expect to gain. Why do you think you are suitable for the role and the company. Also they want you to know BT's products and competitors. The interview during the assessment day, is overall the same with the Skype Interview. For the case study, they send you in advance a subject (which is usually related to BT) and you prepare a 5min presentation. There is one assesor who makes questions at the end of your presentation. The case study was simply a SWOT analysis of a particular scenario.