As at Jan 2022 I have been switched over to BT Digital. My friends and family notified to me that there is no engaged tone when they call me and especially when I am using the phone.
Called BT, didn't get any help and tried to fob me off!! Really, honest truth.
I said this is not good enough and not acceptable, the lady on the phone went away and came and said they will send me a free digital phone which should fix this, that's all she said! Received phone, still same problem! Googled problem and came upon this post which has helped me. There are no instructions anywhere to fix this problem. You can't even change this on your BT Account under digital voice. You have to go in to the digital phone Menu/Phone Settings/Line Settings/Call Settings and Call Mode should be Single call and not Multi Call. This will restore the engaged busy tone.
The thing is, you need a digital phone to enable this.
