Pros
Flexible schedule and remote work culture in many divisions. This flexibility is needed for work-life balance because you may be expected to work with people across the globe at odd times. Excellent benefits. Appears to be "doing the right thing" in environmental concerns, women in leadership roles, paid time off for volunteer efforts.
Cons
Complicated structure of cooperative/competing business units makes for too much bureaucracy and the company is increasingly run by cost-cutting finance teams that are scholarly in management-by-spreadsheet but don't have a bit of experience in the actual service. BT is huge - your experience is very dependent on the quality of management within your division. Career growth can be poor in some geographies but good in others.