So, here's the thing: I
have a 15 year old daughter. If she were actually snatched by a kidnapper and threatened with rape/murder/whatever, I am not absolutely sure that she would remember and execute a "code word" protocol. Especially not a protocol that had the extra measures to help keep it from being subverted in various ways, but maybe not even a very simple protocol.
Not sure enough to feel really comfortable betting her life on it, anyway. Not if we hadn't drilled it on a daily basis for weeks and a weekly basis for months.
It's easy to blank on things when you're adrenalized, say if you've been kidnapped. And it's also easy to blank on things when you're adrenalized because you are hearing the person's voice saying they've been kidnapped.
... and if I asked a scammer pretending to have kidnapped her to let me call her on her phone, I would expect to get the obvious reply: "I threw her phone away. I'm not dumb enough to let you track me/her/us through it". Which is totally credible because that's what a kidnapper should do.
When you get the call, the strong prior probability is that the whole thing is a scam, but that's not so easy to hold onto in a situation like that. And even if you do hold onto it, you will be scared.
Oh, and on edit: Yes, I expect I would keep it together enough to call her on her phone to check, since if she hasn't been kidnapped there's nothing stopping her from answering it. I don't know if I'd expect that of others. But it's also true that if I call her, and nothing is actually wrong, I still expect about a 50-50 response rate because she doesn't hear the thing, has it on mute, or is in school and forced to keep it in her locker, or has let the battery run down, or whatever.