I'm about to get "Alexa Advanced Home Phone" from BT with a new installation. Is it possibly to configure Sipgate on it as well (or instead)?
Did you actually mean "BT Advanced Digital Home Phone with Alexa built in"?
https://www.bt.com/help/user-guides/phones/digital-v...
If so, it looks very much like it will only work with the BT Hub and their digital voice service, rather than open to third party VoIP providers like Sipgate.