Tello has not had full support for iPhones on the new T-Mobile-based plans, with issues with group MMS, visual voicemail, Wi-Fi calling, sometimes LTE, and 5G. Of course, Tello is not alone in this: many/most MVNO's converting from legacy-Sprint to T-Mobile may have the same issue.
Some good news: on Howard Forums, a report about a new Tello SIM that enables nearly all of the missing iPhone features (with the exception of 5G): For now, it only seems to be offered to those who already have iPhones on their lines, and the offer vor a replacement SIM looks like it's being rolled out in stages across subscribers, via an emailed offer.
Link to the original report on the new SIM:
And a subsequent thread with the Tello email:
The backstory is what I think is most intriguing: The missing features on iPhones is because of the lack of a carrier bundle/ Apple-sanctioned APN, paired with Apple's refusal to allow manual APN changes. In most of the cellphone world, for anything other than iPhones, this is a relatively trivial issue: but Apple has arguably taken the path of withholding a functioning APN from providers (and users) based on a requirement to pay a large fee and/or buy a large quantity of iPhones directly from Apple at full price. Historically, Sprint MVNOs didn't have this issue, because they piggy-backed on Sprint's iPhone carrier bundle. A few MVNOs that buy T-Mobile service from a middle-man also sometimes had access to a carrier bundle provided by that middle-man MVNE. But T-Mobile's wholesale/direct agreement with MVNOs does not give the MVNOs access to a T-Mobile-negotiated iPhone carrier bundle as Sprint did.
The issue isn't restricted to T-Mobile MVNOs, either: we're just seeing a spate of them now. (Red Pocket didn't have full iPhone support on their GSMA/AT&T plans for years, until they started selling iPhones on their website.)
The early reports above indicate that the new SIM is not giving access to a new iPhone carrier bundle-- but nearly all features work anyway. If that's true, it kind of looks like Tello has found a way to side-step around Apple's policy of not providing a functional carrier bundle to providers who can't pay to play by including some programming directly on the SIM card.
I don't own an iPhone, and I'm not on Tello, yet I found this post fascinating for some reason!
BTW, I am even more fascinated by KentE's explanation / backstory - KentE, how do you know so much about these things? Is it because you are in a related field by profession, is it just an interest of yours, some combination of the two, something else ...? Just fascinating!
My main phone is an SE 1st gen - I prefer the small form factor, physical home button, and earphone port. For years, I've been going out of my way to use the earliest IOS that will accommodate the apps I use. Currently, that is IOS 11, but I need 12 in case I need it to function on Visible, which requires 12 or later.
I actually go out of my way to find iPhones on eBay that have not been updated, and one of the first things I do when upon receipt is to install TVOS Beta profile to prevent automatic updates of the IOS. My main reason for looking for older IOS and not updating is that older IOS phones generally run faster and don't drain your battery as much. Plus, I don't care if I don't get all new functionalities right away. As long as the apps I use are available I'm good.
A second reason is that I'm a jailbreak aspirant, even though I usually don't follow through. With older IOS, you get more information published about which jailbreak tools would be best used. My experience is that I've only once used with consistency a jailbroken device as a daily driver.
A third reason for not updating is that for a time iPhones running pre-IOS 10.2.1 were very valuable to me for use on Xfinity Mobile by relatives. Pre-IOS 10.2.1 iPhones cannot get LTE data on Xfinity (they can get 3G data but you have to actively switch off the LTE toggle). Those iPhones were important when Xfinity didn't charge you any monthly fee if you didn't use more than 100mb in a given month, so I wanted to do everything I could to ensure my relatives didn't accidentally go over that limit. Now that Xfinity makes you pay a $12/month minimum for 1gb (or $15/month for new accounts) this reason is not so important. Nowadays it's just force of habit to retain older IOS on my relatives' phones and to ensure I don't ever get complaints from them about having difficulty getting used to the new IOS.
An added bonus of iPhones running older IOS is that they always fetch a premium when you resell on eBay. For example, I just sold an iPhone 6 running IOS 9.3.1 that my mom had used for about 2.5 years before my sister gave her a 6s+ (still running 12.3.1 after sitting in a drawer for a while). The 6 was a phone I parked with her in case I wanted to jailbreak it. Instead, I wound up reselling it for about the nominal same price I paid back then.