I applied online. The process took 3 months. I interviewed at BT Group in Dec 2017
Interview
1. Upload CV
2. Situational strengths test
3. Skype strength-based interview
4. Programming test and assesment centre
Based on a number of others' experiences, including mine, I wouldn't get excited until you pass the video interview, on whichever platform you use, e.g. Skype. The majority of the process is actually not handled by BT, but outsourced, up until the assessment centre, and it seems very common that one is turned down at the video interview, even though we all seem to be very sure that we did well.
Not the best experience. I wouldn't put too much effort onto thinking about the process until they email you at each stage. They could have you waiting weeks for a video interview, give a week preparing, ask you very simple questions when it arrives, have you thinking that you've answered very well and then turn you down with an automated response a few minutes later, and there is absolutely no feedback, as others have stated, most probably because it is outsourced.
To reiterate, you are not dealing with BT, nor real people that aren't following protocols and checklists, until the assessment centre. Do not get your hopes up and put too much energy in until then. You really just need to look at questions here, go on the 'about' section, on BT's website, and remember the answers.