I think so. I haven't tried it in the US. I'm not even sure if the option is shown when you aren't roaming. It's also possible that the option is there, but it won't let you authenticate on a domestic carrier's network if a Sprint signal is detected.
Do you have Android? On my Pixel, I go to Settings, Network & Internet, Mobile Network, Advanced, and untoggle Automatically Select Network. It will then do a scan and bring up all the networks it finds. You'll only be able to authenticate on one that Sprint has a roaming agreement with.
It will often give an error at first saying that it can't associate, but it actually does. Not sure why. It starts with GSM, then EDGE, then maybe 15-30 seconds later it goes to HSPA, and finally 15-30 seconds later it ends up on LTE. At least that's how it's behaving for me now in Costa Rica (where Sprint has roaming agreements with all 3 carriers).
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