Reddit reviews ZTE Cymbal Z-320 Unlocked 4G LTE Phone

There's some 4G phones you've missed.

• Sonim also has a newer flip phone available from AT&T and Sprint, the Sonim XP3, which is basically a spec'd down XP5s with an actual headphone jack in flip style.
  
  
• There's an AT&T (and thus GSM-unlockeable) version of the Kyocera DuraXV, the DuraXE. It's basically the same phone as the DuraXV. I'm not sure if AT&T still offers it outside of business accounts, but you can buy them online pretty easy.
  
  
• There's also the Coolpad Snap for T-Mobile, Sprint, Boost, and Virgin Mobile.
  
  
• Consumer Cellular's Doro 7050 is also 4G LTE though it can do literally nothing with it besides VoLTE and MMS. It doesn't even have a web browser. But hey, threaded SMS/MMS!
  
  
• There's various other incarnations of the Alcatel Go Flip/Cingular Flip 2 with LTE for various other prepaid carriers. Cricket Wireless calls it the Quickflip, Tracfone calls it the MyFlip. They're all the exact same phone as the Alcatel Go Flip offered by T-mobile, though.
  
  
• There's the ZTE Cymbal Z320 which is sold under various names (like the Cymbal LTE, Cymbal-G, Cymbal Z - though confusingly not the Cymbal T, which is an Android phone) by various discount prepaid carriers. I'm not sure if this phone is even officially sold by any carrier anymore, but it's readily available on Amazon/Ebay, and you'll probably find it gathering dust in a Walmart aisle in some backwater town somewhere.
  
  
• If you can get one unlocked, the LG Exalt LTE will actually work on AT&T or T-Mobile, since it actually does have the right 3G and 4G GSM bands despite being a Verizon CDMA phone... however, without a Verizon account you won't be able to use the cross-platform feature of its messaging app (which lets you text from a web app synced from your phone, if you want) if that's something of a selling point for you.
  
  Hope this helps.
  
  Edit: Something to note here is that these phones all fall into one of two categories, software-wise. They're either running an extremely crippled fork of AOSP (the Android core) with a few proprietary apps, a locked bootloader, and no support for Google Play Services, or they're running KaiOS, which is web app based linux OS forked from the old FirefoxOS. That's presently the only two ways it's possible to build a VoLTE capable dumbphone, none of the old java OS's can manage it. At the moment the only two KaiOS devices in the US are the Doro 7050 and the Alcatel Go Flip/Quickflip etc. Neither have full KaiOS functionality (or even the most recent version) but the Alcatel devices, depending on the model, may have functional ADB for app sideloading and there's an active community around homebrew apps for the European Nokia 8110, another KaiOS device.

Where exactly did you purchase these phones? For example, if you buy a T-Mobile branded phone from Wal-Mart, those phones are locked to Wal-Mart's Family Mobile network, which although runs off T-Mobile's network, is their own company. Their phones are SIM locked and will not accept a T-Mobile SIM card. You used to be able to do this, but about 2 years ago, they started locking the phones.

Secondly, the Nokia E5 is a dogshit phone for the network these days. No VoLTE and no band 12 leaves a lot to be desired, and 2G is going to be eventually sunset.

Third, does your mom use data or all, or just calling and texting? If so, there's an even cheaper plan for just $20 a month where it's just talk and text. If she's just using a flip phone (and like you said, just wants to make a phone call), then you can have an even cheaper plan than that.

Do yourself a favor and avoid the call centers on the other side of the earth, go to the store, ask if you can change the rate plan to the talk and text, and just buy your mom an Alcatel Go Flip. That or just buy the ZTE Cymbal off of Amazon as it has VoLTE an all the bands T-Mobile uses as well.

Here:

ZTE Cymbal Z-320 Unlocked 4G LTE Phone https://www.amazon.com/dp/B01M4FAY4J/ref=cm_sw_r_cp_apa_i_eLr9AbGFPDXCH

Alcatel GO FLIP 4044 2.8" GSM Unlocked 4GB Blue 4G LTE 2.0MP Wi-Fi Calling (Certified Refurbished) https://www.amazon.com/dp/B07CGXLKCL/ref=cm_sw_r_cp_apa_i_pPr9Ab6KCX9V3

Looks to not have LTE but should connect on 2G until 2020 for calls:
Pantech Breeze 4 (4G LTE) - GSM Unlocked https://www.amazon.com/dp/B00OR1SIJS/ref=cm_sw_r_cp_apa_i_rNr9AbTF9CPRF

Another option, not able to use it on T-Mobile but works on a Verizon MVNO:
Jitterbug Flip Easy-to-Use Cell Phone for Seniors - Red by GreatCall https://www.amazon.com/dp/B01DRV2BBY/ref=cm_sw_r_cp_apa_i_QWr9AbA5Z1444

There are all kinds of options if your willing to look at a different service provider.