I had call sign too when I had a BT line, as the charge was the same whether you had 2 features, 3 or 4.
I was paying for Caller Display and Call Waiting, so added Call Sign and 3-way Calling.
When BT introduced BT Privacy at home and I could get Caller Display for free as part of that service, I dropped Call Sign and 3-way Calling, so just paid for the one item of Call Waiting.