You’d connect the PSTN port of the N300A directly to the FXS port on the (future) BT router. That’s your future “landline” VoIP - but the N300A simply thinks it’s an analogue line.
Then connect the Ethernet port of the N300A to the router for a network connection for your other VoIP account to function off the N300A.
On the N300A setup your other other VoIP account credentials and setup the outbound call selection to favour the VoIP over the “landline”. You can choose to do so for all calls or selectively on a call by call basis - prompted on the handset to choose which in initiating a call.
Use the DECT functionality to add any other Gigaset or third party handsets to the base.