As far as I know Future Fibre does not offer a voice line, so you can't port to that.
Instead, I'd suggest porting the number to a VOIP provider (e.g. Sipgate Basic), which has a one off cost for porting, then depending who you port to can be free or incur a monthly charge.
However, it's important to ensure that you have the new FTTP install in place before porting the number and the port will cease your VM service and you will lose the number.
If the number is not important to you, then you can create a free account with Sipgate Basic and test out their VOIP services to see if that works and then just ditch VM.
In terms of equipment, you could pick up something like the Gigaset N300a box to work with your existing DECT phones. That box would handle the connection to the VOIP provider.
You can also get VOIP apps for phones to make / receive calls using the "landline" number
TalkTalk Future Fibre - 500Mbps / 75Mbps