Pros
Quite often you are left alone to your own devices. If you hate being micro-managed then you will be at home at BT. The managers are all really busy having managers meetings so leave you to get on with what you are there to do. Being such a large organisation, if your face fits then you have the ability to move around the business and can stay with the same employer for years. BT has a good benefits package so you can count on having a shares payout and a good pension.
Cons
BT doesn't know what it wants to do or be. So it lacks identity and focus. Whatever you want to do is unimportant so you will have no support and no backing to get anything done. Processes are so painfully difficult that you can waste most of your life trying to get even the most simple things accomplished. It is almost an entirely 'self-help' organisation because no one will assist you or try and make your life easier. And there will always be someone to block you because your request doesn't fit their idea of easy or logical. It is a massively frustrating place to work. If you need help then you have to search endlessly for it because your management will be too busy doing management calls between themselves to be involved in your day-to-day work. And the chances are that they will have transferred from another part of BT so they won't understand what you need to do anyway. The IT systems are legacy and incapable of supporting a modern environment. Legacy thinking brings everything grinding to a halt. Their propositions are years if not decades out of date. All in all it's a dinosaur that hasn't died yet.