A good question, let me outline the situation in Israel:
20 years ago everyone used the same 2G Nokias, Sonys, etc. Some 15 years ago, with the latest developments in phone technology, namely SMS, cameras and Internet access a bold decision was made by religious (and political) leaders to make a clear distinction between "us "and they" - as expressed in phones.
The decree included two parts: 1. Phone must be from a certified source only (bought through Haredi channels from Haredi suppliers and dealers) 2. A phone must use a distinctive Kosher line starting with special digits (like 05272- or 05276- for Cellcom and 05484- for Partner). A deal was made that Kosher SIM cards (with Kosher numbers) were locked that they can be only used in Kosher phones.
While the first part was logical and supportive, the second was strictly "illegal" as about the same time the Knesset approved two laws of cellphones that, strangely, didn't apply to Kosher phones (I'm not into politics, so I don't know exactly how they did it): 1. all cellphones on the market must be sold unlocked allowing the clients free and painless switch between cell companies and 2. a person owns the number and keeps it when switching companies.
To enforce the order, all Haredi educational institutions from k-gardens to high Yeshivos were/are obligated to reject kids whose parents own non-Kosher phones (clearly seen by the phone numbers). Warning signs were put in all shuls and around the neighborhoods prohibiting entering with a smartphone (strangely the word iPhone (as the pioneer) was used almost exclusively for all smartphones, similarly to the Hebrew "פלאפון" used for all mobile phones, despite being the name of the first mobile provider) see examples here.
The bright side of the deal was that Haredis became a major consumer group which allowed a steep reduction of the prices of Kosher cell plans (with no data of course), usually half of a regular plan (20NIS vs 50NIS). So within a year, all Haredi world switched to those Kosher phones and plans (me included).
The problem began a year or two later when the same [poitical and economical] leaders felt the urgent need to switch to smartphones. That was unbearable, no Rabbi would allow that publicly, so the era of double-life begun - working Haredis bought two phones - one for work and one for their kid's school. The main problem, however, was the fact that smartphones had no filtering or protection whatsoever.
Initially, people were ashamed of being seen with a non-Kosher smartphone in public, but eventually, it became a norm, so another solution was needed and it came very fast - a couple of Haredi companies started to offer special Koshered smartphones also (each for its community):
A regular Android phone (usually Samsung or LG) is rooted, disabling most of its internet functionality and Google services, especially Google Play (installing new apps) and sold for almost twice its price.
A dedicated Kosher app store is maintained by the company allowing the phone to use its apps exclusively. A closed forum of people decides on what apps are allowed to be used (that wreaks havoc, as SMSs are forbidden in Kosher 2G phones and WhatsApp is Kosher in Kosher smartphones).
The list of permitted apps differs between the companies, but usually includes the dialer, SMS, maps, Waze, WhatsApp, camera, banking, medicine, municipal services, e-siddurs and Jewish databases, tools and utils, etc. Many apps are banned just because of their use of [improper] ads. Internet browsers are disallowed.
Everyone can apply for additional apps and usually, it's approved within weeks.
The phone uses the company's VPN to stream all the internet communication thru its services on top of a regular non-Kosher cellular line (with an unKosher number). That is offered as a monthly service for a separate price (usually 50-60NIS a month on top of the regular 20-50NIS plan for 30-100GB of data).
Currently, there are 5 main providers of Kosher smartphones plus Internet Rimon that offers Kosher VPN service for any smartphone (targeting mainly the Kipot Srugot community).
As externally and by number, the new Kosher smartphones are indistinguishable from their undipped siblings besides the Kosher wallpaper, and as the result of numerous problems with Internet access, many wise guys simply buy a regular unlocked Android phone (one that is offered as a Kosher too), put a Kosher wallpaper, use underground services for engraving Kosher stamp on its back and using Rimon VPN כלפי שמיא.
Finally, back to 2G phones, as they eventually vanished from our markets, the Haredis continue to supply them from the third world countries - either old Samsungs and Nokias or branded Chinese phones (see here).