GenMobile used to support Sprint, but now it looks like they are offering service on T-Mobile as well.
We can already confirm that Hello Mobile is already sending out T-Mobile SIM cards for BYOD devices.
We got a T-Mobile SIM card for a BYOD device to upgrade to T-Mobile.
They still sell Sprint Phones, but we just noticed their first T-Mobile Device, the MUV.
It also appears the phones now listed on Hello Mobile support both Sprint and T-Mobile.
Kroger Wireless' Coverage Page now shows a T-Mobile Coverage map, which indicates they have also made the switch to T-Mobile.
They also sell both Sprint and T-Mobile devices.
Madstar Mobile seems to mainly still be on Sprint, but we did see one T-Mobile device in their lineup.
Patriot Mobile shows a Sprint Coverage map with extended roaming, but again we found a T-Mobile device in their lineup.
Tello has announced that they will begin moving their customers to T-Mobile as early as 2021:
They currently still mainly sell Sprint devices and have a Sprint coverage map, however the two devices I personally have owned on Tello also list as compatible with T-Mobile. (According to T-Mobile's IMEI checker.) These devices do have at least minimal support, with even my iPhone showing the main 3 bands needed. (No LTE 66 or 71 though)
I have also noticed that most the devices sold on Tello, even while older, show at lease minimum support for both Sprint and T-Mobile.
They also announced that VoLTE is coming with the migration to T-Mobile. (Hopefully they begin offering wifi calling too)
Twigby seems more lined up to move to Verizon, since they already have Verizon as their main roaming partner and Sprint as their primary.
The big note here is it appears that T-Mobile appears aligned to begin shutting down Sprint's CDMA network as early as mid-2021.
I have noticed most devices being sold are at least partially compatible with T-Mobile or other networks, even if many Sprint MVNO's have not made any announcements regarding the merger.
We are also mainly looking at carriers that were primarily Sprint-only MVNO's since those with agreements with other carriers can simply stop selling Sprint based devices and plans.
Per the Tello announcement, Sprint CDMA service (at least on their end) will work up until at least mid-2021.
"The new network will continue basic CDMA voice service even on older phones until at least mid-2021."
So we may all be moved to T-Mobile rather quickly and Sprint's CDMA service may be turned off as early as 2021, perhaps so T-Mobile may begin offloading those towers to Dish. (Hell, T-Mobile may just simply transfer ownership of the full CDMA network to Dish next year while leaving it turned on. Just that now that it would be owned by Dish, Sprint customers and MVNO's will no longer have access to it.)