Do as Highland76 suggested and port the Talktalk number after the FTTP is live. This ceases the Talktalk broadband and phone.
I really can't see Talktalk residential being able to organise a number export while the service is live.
It's also a service which OpenReach charges the ISP a flat fee for (£35+vat) which Talktalk probably don't have a system in place to pass on to you the customer.
If porting the number to VOIP is a process that takes a couple weeks you could start it before the FTTP goes live and time it to happen a few days after that.
With the higher chance of delays in FTTP installs I would just wait till the service is active then port the number.