1 have you switched on parell mode on the extension ?
if so then you need to confirm that you have an annalogue dialtone on the pair of wires connedted to T+R (connected to the bell ie remove the bell and connect directly to an slt handset
if you have a dialtone then the fault is power supply the LBR may need a capassitor inline these would normaly be built into the BT wall socket but it may have have gone faulty during the change over.
peter (
www.novacomms.net)