Your options are limited.

1. Use the BT Hub if you want to make/receive calls.

2. Continue using your own router and just stop receiving/making landline calls.

3. Migrate the landline number to a VOIP provider, which also ceases your FTTP. You could then order it again as a data only product.

There is no way to make or receive calls without using the BT Hub.
There is no way to migrate the landline number away without it ceasing your FTTP.

Yoy "might" be able to delay the process by claiming you have an alarm that requires dial tone. It would buy you a matter of months though (if the delaying tactic is still even an option).