Pros
There are some great people and teams who work here who are happy to support and encourage you in your role. Great visibility in certain job roles. I find people tend to hired pretty easily after BT. Being in a big business allows you to sample many aspects of if the skills are transferable. BT is in the process of moving to EE - big change and exciting times ahead.
Cons
Be warned: Recently the BT leadership are moving forward with an attitude of "if you don't like the pay, then get another job". They cap rises at 5% within teams so there are huge numbers of people drastically underpaid. As it is a huge company it is a role of the dice of what team you will end up in. I've ended up in teams with management that ruined my mental health and teams which I love being part of. Too many contributors to the same solution. You will spent a lot of time on calls with people will all do a different variation of the same job. Which usually stalls progress rather than encourage it. Parts of the business feel like their job is to work against you from delivering, rather than with you. Their job satisfaction seems to only come from saying no to an idea because of something which happened to their granny in 2008.