tele2 is a carrier pre selection operator which means you route your calls through their system over a bt line. you either dial a pre-fix code before the number you want to dial to or set up CPS with bt to route all calls through them without having to dial a pre-fix code. either way you keep the bt line and pay bt the line rental and you pay tele2 or any other OLO for all calls made through them.
This will not affect your request for adsl with any isp requiring a bt line.
i hope this helps.