Yeah its a bit of a mess, as it always has been if you have your number tied to your ISP. Its why I have always kept it separate even though that was more expensive.
I actually use the "landline" more than I ever have before, as on AAISP VoIP its much cheaper than mobile PAYG, all calls are recorded and the sound quality is better which as I suffer from fullness in the ears particularly in Winter, I've had so much trouble hearing what people were saying on mobile. I've never made enough calls for contracts to make sense.
I suspect the only safe way is to port the number first to an independent provider like AAISP, have a backup plan in case that ceases the FTTP contract. Or get a new FTTP service first, then port the number and cease the old contract. I just wouldn't trust any other way to keep the number.