Cosmo JrTrack 5 suddenly can't call, but can still text

I bought two JrTrack 5s for my kids and we're trying to get the hang of them. They were working well enough, until suddenly my daughter's phone wouldn't make or receive calls. My son's still would do calls for a while, but then it stopped to. Call connecting is already set to optimized (there's no option to turn it off). But phones can still text. Can anyone help me here? If these things are this unreliable, I'm going to return them and try Bark, I guess.