Twice this year I’ve run into issues with Cricket Wireless where I could use Data/Text but I could not make/receive phone calls. Support will just try to gaslight you into saying your phone isn’t supported, which may be true due to AT&T’s disastrous VoLTE network upgrades. I just finished writing this post about my whole ordeal with AT&T and all their sub-carriers:
TLDR; T-Mobile will still support your old devices for now.