[Revelation]

Hi Everyone, we have a BT revelation system installed over 20 years ago which has worked perfectly until on 5th January BT (without our consent) transferred our phones to another rogue company who (without our authority) tried to sake over our phone service.
BT now say that they cannot re-connect as a Revelation requires PSTN and also Featureline which they say is no longer available. I can see from the manuals that PSTN is required but can not see any mention of Featureline.
I spoke to an Openreach engineer in the street yesterday and he confirmed that PSTN was still available in our area but he did not know about Featureline.
Can anyone advise if Featureline is a requirement?

I am unsure if BT really can not simply reconnect or they are trying to sell me a new cloud based system
Antony