There are three carrier networks.... AT&T, Verizon, and T-Mobile.
There are also "now" three types of networks using 4G (LTE), 5G, and mebbe 5GUC (for some devices).
Something is "special" and there is no "one-size-fit-all"...
My guess ... the flip is 4G and the others 5G (and not negotiating)... or it's raining ...???
I dunno
HTH ....
edited: Toby ... it might not be their phones, etc. The various networks might be flakey due to other factors at times too.
I have to juggle mine all the time fwiw...