I wasn’t much concerned with the source of the line, actually. In the US it is common to do back-to-back setups like this because many residential subscribers get bundled Internet+phone+cellular plans that make the “land line” cheaper than a standalone SIP trunk (I understand SIP has no concept of a trunk or a line and trunk usage is more common when discussing business telecom especially DIDs) and the handoff is a POTS port off some fiber-to-ethernet converter box. In other cases, such as Vonage SIP “lines” the delivery is ONLY to the Vonage ethernet-to-FXS box and the handoff is POTS. Vonage refuses to deliver SIP direct to the customer even though they deliver SIP to their own hub so you end up with the back-to-back silliness. An even worse one is MagicJack where they didn’t even supply a proper hub for a long time they required an entire computer devoted to servicing a USB stick. Recent iterations of MagicJack eliminated this need as they now have both a POTS port and an Ethernet port although they retain the USB stick form factor and only use the USB port as a power feed into the stick.
I believe magickJack is cheaper than most SIP trunk/line providers in the US. In the US unlike the UK, the telephone company was never nationalized but instead it was a regulated monopoly and still to this day the states regulate residential phone service, thus a residential POTS line is 50% the cost of a business POTS line. The states and municipalities also apply a LARGE amount of local taxes to telephone service. And to add to the fun the local counties (separate from the cities and states) have 911 call handling and so apply their own 911 taxes. There is no law in the US mandating the phone companies get rid of copper lines, however. Communication law in the IS is very complicated because it is all built off regulations introduced on a national level years ago that have survived court challenge and as a result nobody in government wishes to make any modifications they just keep layering requlations on top of the existing ones. But fundamentally the FCC divided communications services into “entertainment/regulated” and “unregulated” Cable TV/Telco was regulated, Internet was not. This permitted the government to enforce “sin laws” against nudity, etc. on the cable TV network and exempt the “paid cable TV” services, all in the name of “protecting the children” Unregulated were paid cable TV, regulated were telephone and “basic cable” basically allowing counties to force cable tv providers in low terrestrial tv signal broadcast areas to provide retransmission of terrestrial broadcast tv signals (which were content regulated, no porn) Then when Internet service came along the goody-goodies went ballistic, also wanting to regulate all that nasty nudity but the Internet providers used the unregulated loophole to get their services classified as unregulated. So that is why we can say naughty words on the Internet in the US without the government censors blocking them out. Interestngly, when Vonage/MagicJack and other “consumer” SIP/SIPPOTS services became a thing, there was a movement by those companies to make the claim “my voice service SIP is delivered over data, it’s packets, therefore it’s unregulated” so they could duck paying all the municipal taxes, but the state/county/federal bureaucracies, colluded together with the courts to squash that, recognizing what a Pandora’s box it would open - so those services knuckled under and pay their voice municipal taxes as well. Cellular services, OTOH - those learned from that example and used a different loophole and the municipalities are still pissed off about losing that tax revenue.
The US also has a similar law, number portability. However, we still do have telephone numbers that are non-portable. DID pilots and DID numbers tied to an exchange sometimes are non-portable and there’s certain NPA/NPXX prefixes that are not portable. I do not know what the reasoning is or the law exception that allows it, but I know it is a thing.
In many places in the US the carrier has replaced distribution lines with fiber then they have a fiber termination at the pole which uses the original analog lines to deliver Very High Speed DSL to a CPE hub at the customer residence which hands off via POTS and Ethernet. In other places they still do have POTS all the way back to the CO. I have a home with one of these for example because years ago I got an ISDN line. (I no longer have it) ISDN is not a service they ever delivered off a fiber box in the street so that line still exists that is direct to the CO even though it’s POTS only, now. And still in other locations the phone company operates a mixed network of copper POTS and fiber to the home - and as long as the subscriber maintains their original copper POTS line they are left alone but if they switch to phone+internet, their copper line is decommissioned and permanently replaced by fiber.
There are other oddities. For example no Internet provider in the US will deliver a static IP address to a residential Internet service. You must switch to a business class Internet service which costs 2-3 times as much even if you are a residence. However there is no law regulating Internet pricing the way there is basic telephone service. Internet service providers make use of the notion that there is such a law in order to get extra profits with higher prices from businesses.
Lastly this back-to-back discussion isn’t really complete without mentioning Quality of Service. With companies like Vonage, MagicJack and plain old SIP trunks/SIP lines - because these services are delivered over the customer Internet connection, and they are NOT delivered by the ISP that is supplying Internet connectivity, they do not have packet prioritization applied to their traffic - so if the customers Internet line is congested - call quality can go to hell.
However, if the customer gets phone service from the ISP even though the service is VoIP terminated into a POTS handoff, then since Internet and voice are provided by the same provider, the provider prioritizes voice service with QoS. Their CPE device respects and participates in this packet queueing thus call quality is maintained no matter what the Internet network is doing.
For example in my area we have Comcast cable. Comcast charges $12 a month rent if you use their cablemodem but for residential users they do not require you to rent their modem - unless you also get voice from them. So if you have Comcast, your choice is money saving, with your own modem, and using some provider like MagicJack which has unprioritized SIP-into-a-locked-hub with POTS handoff and crappy call quality, or expensive - with Comcast’s modem you must rent, and their own prioritized POTS handoff that has excellent call quality. So excellent in fact, that you can run a FAX over it. (talk about another dinosaur technology) i should also mention that both Comcast’s voice and any 3rd party over the Internet SIP provider’s voice is required to charge the customer any local municipality taxes as well as 911 service fees.
So there are, in fact, other considerations that someone in the US at any rate, might do the back-to-back thing, as technically ridiculous as it appears.