They're probably not listed in the BT Shop to stop customers inadvertently buying them. BT would want to be certain that the customer does have the DV service before selling them.
Had the same problem. Full fibre installed but no "free" phone sent. Did not initially realise I was due one but someone eventually mentioned it and I ordered a pack of four. They duly arrived and after some head scratching realised they had sent "Essential" phones that were not digital. Called BT and there is, or was, confusion. Initially, a young lady told me I was only allowed one Advanced phone or two Essential phones but could only order more from the BT Shop or by doing a Google. I pointed out to her that the BT Shop did not have any and that a Google came up blank. Eventually I was transferred to a gentleman who knew more about it and I was able to order four Advanced (although he had trouble finding out what the difference between the two was - mainly a coloured screen which was enough for me to pay the little extra).
The phones arrived quickly although by then I had managed to get our five Panasonic phones working (don't ask!) Sometime later the Panasonics played up and I decided to connect the BT VOIP phones. No problem there but I later decided to try the Panasonics by plugging them into the split connector on the router, HH2. Lo and behold we had nine working phones! That was then and it lasted for a couple of weeks until a few days ago when we started getting reports that we were not answering our phone from landlines. I have tested the line from my iPhone, the wife's and son's. All works as it should. How there is a problem from landlines I cannot quite understand and have yet to contact BT about it - lack of time. Pleased to say the Fast Fibre Broadband has been faultless!
So here we are some six weeks later and the problem is still not resolved. I have had many conversations with some lovely BT staff who promised to take responsibility and it sounded as though there were some serious conversations taking place with people higher up. On the BT site tracking faults or complaints (and some of that needs sorting out) my VOIP was connected on the 19 November. Yet again a broken promise. Quite simply people cannot call in from a landline, it just rings out without ringing a house phone, but we can receive calls from mobiles which go to 1571 if unanswered. Despite several comments on the complaints page I have yet to hear more from anyone at BT. There is much more to this that would take far too long to write such as the engineer who was sent to sort it out was a "copper" engineer with 30 years experience but no knowledge of fibre. There was the time I was transferred to a fibre expert who the proceeded to try and flog me wifi discs. Not forgetting all this started when the fast fibre was installed in August.
To cap it all, I found out yesterday I could not cast BT Sport again from my iPhone. The last time that happened it turned out some bright spark had changed my contract to BT Sport mobile only.
Is there another provider who knows what they are doing?
Do you still have all the panasonic phones connected? If so have you tried disconnecting Panasonic base from the hub and therefore disconnecting the Panasonic phones and see if that improves things