My calls are automatically routed through another provider, but I pay line rental to BT. Can I still use 1899?
To have all your calls automatically routed through another provider is also known as Carrier Pre-Select (CPS). You will still be able to use 1899 as usual and this will not affect your CPS subscription. Just sign up and dial 1899 followed by the telephone number to make cheaper calls! Your CPS provider will not charge you for dialling the 1899 code, therefore the 1899 code will not appear on your CPS bill and any calls made using the 1899 code will only be invoiced by 1899.