I'm trying to setup a fixed landline through my Fritz!Box 7390

I have a custom Y-splitter cable connected using an old BT to RJ11 cable.
On the VDSL side I have pin 2 & 6 of the RJ11 jack connected to pin 4 & 5 of the RJ45 jack with VDSL connected and working fine.

Then on the phone side I have pin 2 & 5 of the BT jack connected to pin 6 & 3 of the RJ45 jack and its showing up as connected in the Fritzbox

I have two Panasonic DECT KX-TG6721NZ's and an analogue phone connected to the Fritzbox on Fon1

When I call the landline all the phones ring but when I answer the call I get a busy dialtone and the mobile keeps ringing
I have a solid dialtone when trying to call out, but when I try a number I get a busy dialtone again.

Any help would be really appreciated thanks