he must be
a single person with unlimited budget and can not find the off button on his mobile. Everyone I know, all ages, prefers to use their land line when possible: cheaper, often better reception and anyone in the dwelling can answer the 'phone, with no problem over reception because the flat is covered in several floors of concrete, or the old house is in a hollow and made of granite or the battery has gone flat ....
Regarding mobiles and dialling area/supplier codes: I suspect that most of us ringing from a mobile ring numbers stored in the mobile most of the time and store the international prefix too as, it being a mobile, one may use it abroad. I suspect most private individuals rarely "dial" a number (except to save it to the 'phone memory) other than to ring a shop or some such and, even then, if it is the barber or the dentist, they probably store that too. Even on the landline, as the 'phone can store a couple of hundred numbers, I rarely dial a number as all my usual ones are stored in the device, whether at work or at home.
As for just extending numbers: quite apart from the software changes and configuration costs in private and public systems, the cost is unbelievable for the publicity/education alone, the organisation and so on. And never forget, the vast majority of the population use "technology" in all their devices in just the same way they use a kettle or a microwave - not a thought as to the technology behind it or even that it is a technical device and so no idea of more than the simplest functionality and no interest either as, like a kettle, all technology is just a means to get a job done so you can get on with the important things in life. Judging by many of the "informed" comment on these pages, even so-called technically aware people in reality do the same, knowing the jargon and not the technology.
In fact, as with all customer interfaces, if the average person does have to be aware and concerned, it is a bad interface - hence the success of Apple through providing the easiest interfaces for what most people want to do most of the time. The nerds who want to build their own 'phones, cars, computers can do that as their hobby while the rest of us just buy a device that works and get on with life. if it is too awkward to use, we get replace it or have the sense not to buy it in the first place (hence the failure of Linux to penetrate the mass market; I write that as a UNIX specialist who loves command line, pipes, filters....). This is why the way to use a telephone, in all its forms, has not changed fundamentally since its inception, within a few minutes of first acquaintance, it is easy to use for all ages and mental abilities and stays reasonably consistent across decades, even centuries.