Hi, I have just got my Grandstream HT801 working, and have been able to resolve this exact problem. I am assuming from your posting that you wish to retain use of any existing extension BT phone sockets around the house? If this is NOT the case - as others have suggested, just connect the base station of a set of wireless phones to the phone socket on router and then put the extensions where you want them.
In my case, I wanted to use my own Netgear D7800 router AND retain the landline AND STILL have access to the house extension phones. This works for my setup, but Icannot see why it wouldn't work for the original Vodafone router or indeed any other.
As you will have found, you need an RJ11 plug to 431A (or 631A socket ( actually they are the same in appearance - just that one has only 4 of the wires connected and the other 6) - which gives you a 'Standard BT phone socket on the free end enabling you to plug in your original BT type landline phone. BTW, assuming you got this adapter cable from Vodafone, you will first need to plug into this socket one of your line filters - NOT because the line actually NEEDS filtering - but the Vodafone adapters do not contain the required capacitor, so any phones plugged directly into them will not ring! (Found this the hard way!)
I then bought from Ebay a standard BT socket phone splitter and a phone extension lead with a male plug on each end (a bit more elusive than the usual extension wire, but inexpensive from Ebay). I then plugged the socket splitter into the BT phone socket on my wall, and plugged my first telephone into one of the available sockets. The male to male extension lead was plugged into the other socket at one end and the available socket on the line filter that was connected via the short adapter provided by Vodafone, into the phone socket of my Grandstream HT801 phone adapter - although I cannot see that it would not work if plugged directly into the Tel1 socket on the back of the Vodafone supplied router.
Once this has been done, and BT extension phone sockets around your house should start working again as they did with the old (analog) telephopne system connected over the copper wire connection to your house.
I would just say that this should ONLY be done once you have got your new phoneline which is supplied over the fibre connection, as once this has been done, Openreach will cut off the copper wire connection from your house back to their equipment - so there is no risk of the two interfering with one another.
