Just to state my experience which I trust will be helpful to those having recently acquired a B525 with firmware 11.190.61.01.1217. Firmware commonly installed on EE and other network supplied routers. This is locked to 4G mode so connecting a DECT phone will not work as it needs to drop down to 3G to take receive calls.
Get a copy of the readily available 11.189.63.00.1217 bin. With your router connected to PC via LAN cable do a local update. Find the bin file on the PC open up and run the update. No need for USB to USB, opening up router etc. On successful update you will see that under modes it now says Auto or 4G, 3G , 2G selection, when before was just Auto or 4G options. Plug in your phone and hew presto it now takes calls by dropping down to 3G.
Prepared to be shot down as new to the table but why cannot local updates via LAN be more widely used as opposed to USB to USB?