When I moved from BT to Virgin I had them put their phone socket next to the BT master socket. My extension wiring terminates in a junction box next to them and has a short patch cable with a BT plug so that I can easily plug the extensions into either BT or Virgin. Virgin did want to connect their line direct to the BT master socket but I wouldn't let them, I insisted on a separate Virgin phone socket. I assume they would have left the BT cable unterminated!

And probably cut the lead in at the entry point just to make sure