To sign in to "My Account" on Talktalk you use an email address you provided them. I used my Gmail for this.
To sign up for FTTP while keeping the FTTC running you need to setup a new account with Talktalk, with a different email address.
That is unless they have changed things very recently and now support both services on the same account. They certainly didn't when FTTP launched.
They had the same limitation if you wanted 2 x FTTC lines. They had to be separate accounts.
I presume I could if needed move my VOIP number to any provider at a later stage if I wanted to.
As long as the VOIP is not part of a
bundled broadband+voice service, yes.
The same applies with BT's (and I believe Sky's) VOIP as above. If you migrate the VOIP number the broadband ceases.
That's why I think it's best separating the 2.
It's different if you take a data only FTTP package from a provider, and separately take VOIP.
If it's a bundled package though migrating the VOIP number ceases the broadband.
If you want both broadband and VOIP from a single provider then you simply need to migrate your number to their bundled FTTP+VOIP package as a standard migration.
The Talktalk FTTC would cease the same day the FTTP went live with the new provider.