I'll let you know when I port mine.
Did the FTTC to FTTP first, now trying to figure out "when its safe" to port the POTS to VoIP.
From what I can gather so far, it takes a while for the account details to sort themselves out as the FTTC is ceased and the databases are updated so the account no longer is linked to your landline. Yesterday it was still showing my telephone number against my account and Bundle: Yes, now that appears to have gone but the order hasn't closed yet. The physical connection itself was done on Monday.
NOTE: This only works if you have line rental and broadband with different providers. If they are both with the same provider it seems to be hit and miss, but is supposed to work if you move them over to the FTTP + VoIP package, you can't split them to different providers without losing your number though should be able to port VoIP to VoIP later, but presumably only once the minimum contract is up.