Great fun - tell them you found out that your computer was “blocked” (Removed). They block a lot of VoIP services but you can ring them via Skype. Their filtering on their call centre software is a bit rubbish so just keep redialing and you’ll pop through (even with no number).
If you wind them up, offer them a job, ask them what they're doing with their life they get very engaged and defensive.
Good fun to be had!