Yeah the one more visit would be for the engineer to do a test and also phone back with the serial number of the ONT so it knows to route your traffic to your ONT to decrypt.
As for them to install their own FTTP fibre I think that is rubbish, if that was the case then nobody would have FTTP when they move providers.
Also how would it work if you have more than one FTTP connection with different ISP, it uses the same fibre cable going into the ONT, so what they said doesn't make sense.
The way I see it FTTP is FTTP end of.
If the ISP are stating that then they have no clue or are confused.
My guesses they are confused.
I assume it was BT that said all this information.
What phone number did you call at BT?
I use 0800 587 4787 and have yet to have issues.