Looks like theres another issue:
If you are with Plusnet and you want to upgrade to FTTP then you will lose your landline number ( as Plusnet will not provide FTTP customers with a landline)
and for those who use plusnet email addresses , if you move to another provider then you loose your email addresses / access to plusnet email as plusnet have stopped providing email only accounts
The way to handle the landline would be to order the FTTP as a new/separate service (rather than an upgrade / migration from the existing FTTC based service). Once the FTTP service was successfully up and running, then and only then initiate a number porting request with a third-party VoIP provider. This has the effect as we know of ceasing the copper.
Importantly the landline number is retained (and can be utilised on ANY internet service anywhere) AND there is no drop or disruption to broadband. The (small) downside is a limited period of dual running both FTTP and copper services.
In some ways voice should now be treated as a portable asset able to be migrated at will independently of the broadband provider one uses. Makes life much easier to move at will.
As you’ve opened another Plusnet account, this ought to retain your email etc - but note the advice above from dect.