Telstra one number and Apple Watch family set up

My mother in law, who is disabled, recently got an Apple Watch SE for the purpose of being able to answer and make calls when she is away from her mobile and also for voice activation incase she has a fall.

She doesn’t have a smart phone and has no photo ID so I have set up another post paid service on my Telstra account for her and plan to manage the Watch through my iPhone 12. At the time of doing this I was told in the Telstra shop that I would be able to set up the Apple Watch through family set up and assign her new phone number to the watch.

I have tried so many times to make this work and went into a different Telstra shop today for assistance and was told that it won’t work with family set up.

Does anyone know how I can assign Telstra one number to this phone number so I can use it with the Apple Watch?

thanks in advance.