Pros
It's a big company so there are endless opportunities to move around into different lines of business and to go for promotions, which from personel experience I can say are very fair for everyone. I started in BT back in 2006 and made sure I showed promise and a willingness to be keen. Within 6 months I moved onto a more technical team and within another 2 months I was given the chance to learn the ropes of being a manger. Which I did and I was acting manager for my group while our boss was on holiday. Another good reason is that you feel like your doing something worth while. It's a good feeling when you help people and they are greatful for it. Also its like a family here. Everyone says hello to everyone and all BT people are more than willing to help, even if it means it puts them out and then they have to work harder to catch up! And then there are all the benefits/discounts from other companies that BT people get. So all in all, pretty much the best firm I've worked for!
Cons
It can be too big and you feel like a number, so not much reconigntion in the form of bonus/money.