On past performance, BT are well-known for screwing up existing connections when connecting new.
Yes. Indeed.
One office I know was having a simple new copper voice line put in for a specific use (nothing fancy, just a single pair PSTN line like you have at home).
What did BT go and do ? They went and disconnected a live LLU ADSL service that was being delivered to the same site, and re-used the wires to provide the new line.
Despite two way email exchanges with various BT board members, it still took 10 days before they got round to fixing it .... and even that took two attempts since the first engineer they sent out was utterly incompetent. Of course no apology for the inconvenience was ever provided despite it blatantly being a BT problem.