Thanks for your very informative comments & links Goslow.
Using those, together with the testing Ive just done today, I think I can now write the definitive version of how the Motorola FW500 Emergency Back Up Line (EBUL V3) works and how it can be connected to the Router if you so wish, although it works just fine if you leave it unconnected, (but still powered from its mains adaptor).
So, the VM EBUL v3 is a Motorola FW 500 unit that contains a sim card supplied by VM that can operate on a multitude of mobile networks. The Sim card number is usually on a sticker supplied in its box - (handy to stick this to the phone somewhere)
You have it permanently connected via its mains adaptor to keep it charged up, & in the event of a power loss youve got at least 8hrs of call time - or far more if just left in standby.
It has a single phone cable connected to it which is terminated with a regular BT 431A phone plug. If you want to connect it to the RJ11 'Tel1' socket of your VM router you'll need an RJ11 to BT plug adaptor like this. And if this RJ11 socket is already occupied by a cable to your landline phones, then you'll also need a line splitter adaptor like this so that both RJ11 cables can be connected to theTel1 socket.
If the EBULv3 is used UNCONNECTED to the router, then it can be used at ANY time to ring ANY UK number whether it be Mobile (starting 07), Landline (starting 01 / 02 / 03), Freephone, or Emergency numbers. It uses its built in SIM card to connect to the network and the person receiving the call will see the SIM number of the EBUL (starting 07). Anybody can call the EBUL phone if they have its SIM number and ONLY the EBUL phone will ring. When used unconnected to the router you may not hear a regular dial tone from its handset when picked up. All calls made via the EBUL, ( to the above UK numbers), will be FREE of charge as long as those types of call are normally part of your VM call plan.
If the EBULv3 is used CONNECTED to the router (whilst the router is working normally), then it assumes your regular landline phone number & becomes part of your landline array - and so will ring & can be answered if anyone calls your landline. If you make calls from it then anyone receiving those calls will see your regular landline number displayed. All calls from it will be charged the same as any other landline phone in your house according to your VM phone plan. As soon the internet goes down or you have a power outage, the EBUL will detect the lack of signal on the Tel1 connection & automatically switch to SIM card mode as described above, reverting back to landline mode when the internet signal is restored.
I just wish the info that VM tells its engineers & moderators on here about the Ebul v3 was brought up to date, as most of the info they were telling me including the VM links at the start of this thread was just wrong & very unhelpful.
