The BT handsets should be GAP compatible and so should pair with the N300 if you wanted to use them for 3rd party VoIP calls.

I have been unable to register the BT digital phone with the Gigaset hub. I am wondering if the BT phone is restricted to the BT Smarthub 2

What brand is the phone? Yealink will not register with others - BT use those for business, or it may be a specific firmware buld which precludes it.

Hub is a Siemens Gigaset N300. Phone is a BT branded digital phone.

The N300 series are all very good at working with most handsets. However, some manufacturers look their phones to their own base stations - and that could be what you have.

It is te underlying brand that is need - not just the BT label. Have a look under tehbattery to see if there is a part number or manufacturer ID>

There is no branding on the BT Digital Phone which suggests that it is not a rebranded phone. It comes as no surprise that they use their own firmware that does not allow their hardware to be used with other manufacturers. I have therefore concluded that it is not DECT compatible.

The BT Digital Home Phones claim to be GAP compatible so should at least be able to be used for basic phone functions on other DECT base stations.

But if it doesn't connect to the Gigaset base station?