Closing down a BT account.

[-=BigD=-]

I've just rang to cancel my BT residential account as the company I work for has installed a BT Business line for me to work from home from.
Now as I was cancelling due to having two BT lines they said they can do it instantly, I'm waiting for a week to I can tell the handful of people who ring me on the landline. But they mentioned that if I was moving to another supplier there would be a 30 day cooling period until it's cancelled. Why???
If I signed up for another, non-BT, residential line over the next few weeks what would happen?
TIA ABers

[koster]

I signed up to change my residential line from BT to Yourcalls.net on 10th December. The changeover took place on 29th December - that's less than 30 days!

By the way, if you're changing, it's worth pointing out that Yourcalls.net charges extra for caller display. With BT it's included in the line rental.