Orbi Mesh System with BT Phone

Did you transition from the previous BT analog phone service to their new digital voice service as this person did?

https://www.youtube.com/watch?v=Na-Ie90EeY4 

Other Internet Service Providers, such as my Spectrum service deploy modems with analog phone jacks.  Since my phone connection is to the modem, it is entirely independent of whatever router is attached to the modem.  Even though your Huawei modem includes a phone jack, it appears that BT has elected to incorporate phone service into their router rather than the modem.

It's a bit confusing, since the actual phone instrument talks to the router using DECT, but appears to sync to the router using the typical WiFi WPS sync button.

You absolutely cannot connect two routers to the single Huawei Ethernet port.

There appear to be few strategies to (a) keep the BT phone service and (b) use the Orbi WiFi.  For example:

The BT Hub 2 can be set to disable its own WiFi signal.

The Orbi 960 can be placed into Access Point (AP) mode.

Now there is only one "router" (no Double NAT problems. port forwarding can be set up. etc.)
But, many of those enticing Netgear features will not work when the Orbi is in AP mode (Armor, Parental Controls, separation of primary/guest WiFi networks, etc.)

If you want to keep Orbi router features and avoid Double NAT issues, then abandoning BT phone service in favor of some VoIIP phone solution would appear to be the only option.

You might get some helpful information by querying people who have BT service.