The Call Sign feature was implemented within the existing physical PSTN equipment in the exchange. As this is being de-commissioned, that service is being ceased, and its not possible to implement a similar feature on Digital Voice, as the ringing current for physically connected phones, comes from the Smart Hub 2 on receipt of an incoming call.
The BT supplied phones, like most DECT phones, do not support it anyway, however for many phones, its possible to assign a different ringtone to specific incoming phone numbers.
I hope/assume there is an option to port my Call Sign number to another BT account or another provider account at least?
To add to this, communication providers will want to withdraw features that were specifically implemented for the current industry wide PSTN technology that is used today as this is being withdrawn in 2025 with a nationwide stop sell in September 2023. So it's of no surprise that call sign is being one of them.
As long as you inform people who use your other "virtual" number, that they will need to call your main number in future, it should not be a problem, which is why BT have given you plenty of notice.
As @jac_95 has said, it was a purely PSTN exchange function which modified the ringing current timing which was sent from the exchange line card. It has no digital equivalent.
There are probably very few BT customers that use ‘call sign’ , it’s a little unrealistic to expect any time, energy , or expense to be spent on what at best is a niche feature being inter-grated into DV if it was hardly used in the PSTN network.
As already explained, with a PSTN switch , the caydence of the ringtone sent from the exchange (which is an alternating electrical current around 90v applied to the line ) changes depending on which number was called, normal ringtone …. ring-ring , pause , ring-ring , or ring,ring,ring,ring for the call sign number, the ‘sound’ the phone makes indicates which number was being called.
An optical network doesn’t have any voltage , the ‘signal’ for the phone to ring , is simply a digital signal that the device responds to.
You ask , as a telephony only customer , why is it being removed now , when you could potentially benefit from it until your individual traditional exchange is closed ?, but the fact is , it’s a facility that no provider has an obligation to provide, and BT have decided to remove it, giving you fair warning of its removal