I wonder. Did someone take their tape measure out and decided how large a phone "ought" to be before it is "allowed" on the network?
I have had cell phones since they first came out, those bricks you'd put in your bag with a long aerial sticking out. My previous phone was a 6.5" phablet, and before that a 5.5", and before that 4.5" and smaller - you need the screen size to be able to see what's happening especially as you're getting older.Phones are getting larger it is the trend not only because people's eyesight deteriorates but also because you need the screen space to fit all those apps. Leading manufacturers are well into the 6" phones now and Samsung include a SIM slot on many (if not all) of their "notes" and "tablets" and you can make and receive calls and texts, so there is no distinction between a "tablet" and a "phone" other than the external size, everything else is more or less the same, CPU, RAM, OS (Android most of the time)... My current phone (Samsung Galaxy Tab S 10.5 LTE) is 10.5" and I can read it much better than the phones I had before.
If anything those ridiculous terms are ageist : they are preventing me from using a screen that I like, a screen that I can read easily and conveniently.
Of course there are workarounds, the easiest is not to go with fascist companies such as Virgin (and Three), luckily there is some competition left in the UK... Another solution would be to get a smaller phone and then tether your tablet to it, if your fascist provider allows you to tether that is. Of course this is inconvenient as then you have to carry more devices and remember to charge and manage two devices instead of just one.