Cost benefit analysis. We are talking about relatively small sums on an individual basis. Collectively it could be a tidy sum, but pursuing to the extent of detailing every single call, for every single user, is futile.You would have to ask Revenue that? Why don't they chase money that should be flowing into the government's coffers?
A more rational approach is to allow a 'reasonable' deduction from a fixed price plan. Don't you agree?
The point being, if I pay €60 a month for a business mobile plan and Revenue AGREE that I can deduct €60 as a 'reasonable' amount, I would be stupid not to claim the full €60 as a business expense even if I also use the phones for personal use. Wouldn't it be stupid not to claim the full €60?