If you manage to get a 4 port ONT it would give the option using a 2nd port for a low bandwidth (0.5Mb/0.5Mb) FTTP VOIP service.
It's designed for anyone with FTTP who wants a landline without broadband, but nothing stopping you using it on another port to your broadband service.
I'm not sure how many ISP's sell this.
BT retail can only provide FTTP broadband on Port 1.
No idea if their low bandwidth VOIP option works on other ports.
I'd be surprised if BT ever sell VOIP over a rivals broadband connection.
If they don't it would mean anyone who currently has broadband from 1 provider and landline from BT, would not have that option with a 1 port ONT.
It would need a 2nd ONT which for many homes there is no capacity for a 2nd fibre/ONT on their CBT or splitter.