Even if you do sign up online your don't always get what you clicked for. A friend went for a promotion which had 10GB/m doubled in the promotion and then doubled again for £10pm. He got his 40GB OK, but the direct debit took at £15pm.
Beware of promotional extras. I allowed my BT Sport to continue for a month in 2019 because the teams in both European footie finals were all English. At the end of the month I rang up to cancel, and was offered continuation at £3 per month, so I accepted. Sixteen days later BT hiked the price to £7.50p - so I immediately cancelled.