0800 800 150
But how do you know it was BT who called? You may have been conned.
Strange they have started the migration w/out your bank details and written confirmation.
Who do you have your phone with? If BT, you can check on your BT online a/c.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC