You seem reticent to state who your new provider is , and if they are delivering its services using an Alternative FTTP Network , or the new ISP is also using Openreach network , wether it’s FTTC or FTTP is relatively insignificant, given that you ask what is the process for leaving BT , it’s does make a difference if the new provider is using OR (as was the BT service ) or not , when it comes to serving notice to BT .
There is a process when moving between providers on Openreach that doesn’t exist if moving to an Alternative Network and the Openreach network becomes redundant , if moving provider on Openreach network the new provider serves notice on your behalf when they arrange Openreach to complete whatever works are required….the way you are describing your situation doesn’t seem to align with either scenario, who is your new provider and over what network are they providing your service ?
There is no issue with either new broadband provider as is not over the open reach network - the problem lies with BT not cancelling my phone line when it was ported to the new provider.
I am not going to share details of the either providers as (a) the broadband is totally separate and has no bearing on the issue and (b) the Voip provider (and BT) have confirmed that the new provider has completed the work.
So who contacted BT to advise them you were leaving ? , you directly by asking BT for your line ( phone and broadband) to be ceased or your VoIP provider , if it were the VoIP provider asking for your phone number to be ported , that has the effect of causing the cessation of the associated broadband , in either case it’s a 30 day notice period from when BT were advised.
BT ( and Openreach ) have no interest or knowledge if the work required to install your FTTP service from an Alternative Network is completed or not , it makes no difference to them , you or the VoIP provider effectively gave 30 days notice to for BT service to stop, the Alt Net have no reason to contact BT , they cannot serve notice on your behalf.
It’s irrelevant if the Alt Net FTTP broadband was activated the same day as the BT service stopped , or if it was before, or after, it makes no difference to BT when your Alt Net was provided, the date the phone number moves does impact BT and the Voip provider
The ‘sorry you are leaving’ notification must have been as a result of the VoIP provider asking for the phone number as the FTTP Alt Net don’t advise BT of anything, unless they are both the network and voice provider and then it’s the request for the number port that BT are responding to .
As stated , if whatever date your VoIP provider asked for the number to be ported , you still ‘pay’ for 30 days after that , if the port request was the point at which notice was served , you shouldn’t pay after those 30 days have expired but the final bill may cross two billing periods so paying and being refunded for days after the supply has ended is normal.