I'd say this is as clear as mud, but I can give you my experience.
I have just terminated my phone+FTTC service with Plusnet, by transferring my phone number to Sipgate.
My Plusnet contract was due to end on 17th August, and I was fully paid up to that date (that is, my last bill was for the period 18th July to 17th August).
Plusnet E-mailed me on 28th July to say that the transfer was going ahead, and it took place on 2nd August.
The mail said:
Because you're moving your broadband and phone before the end of your minimum term, you will be subject to early termination charges. These have been calculated at
£5.15. You can find out more information about this in our
Price Guide.
I've no idea how they calculated that figure, given that I'd already paid to the end of the contract. Plusnet
T&Cs say they require a 14 day notice period, which would take me to 10th August from when they were notified, but I'd still paid beyond that point.
My normal monthly bill was £21.74, consisting of £22.99 minus £1.25 referral discount. It was due to go up to £41.75 after the end of the contract.
My best guess is that the £5.15 is for the period from 2nd or 10th August (end of service or end of notice period) to 17th August (end of contract), which I've already paid. They haven't actually asked me to pay this amount, so I'll wait to see if they do. I might even get a tiny refund of what I've paid less £5.15.
My advice is: choose a date for migration of the service with your new provider which is at least 14/30 days in the future (as per your current provider's T&Cs),
and preferably just before your current contract ends.