While I think about it, you can get RJ11 to BT 431 adapters. Take a look on Amazon. (Sorry, we are not allowed to post links to commercial sites, otherwise I'd point you in the right direction). As I said, though, the real issue is who is providing the phone service and that BT's DV will not work through third-party equipment.
According to Fleur's documentation, there should have been an adapter included with the router. They refer to it as a VOIP adapter & it should look something like this:
Check the box & if it's missing, give them a call.
It's simply an RJ11- BT431 adapter. According to Fleur's documentation, the phone port on their router is RJ11 rather than BT431.
I thought I’d said all this earlier but just to summarize: