I do tech support for cell phones as my job. If it is ringing once, it's probably a phone setting. It's pretty easy to accidentally tick that setting off on some phones, depending on the operating system and maufacturer.
If as far as you know, everything should be normal between youand Sadie, I think it's worth sending her a text saying "hey, all my calls are going to voicemail after one ring. Just wanted to say Merry Christmas". That way, if it was accidental, and she doesn't know where to find the setting, chances are she'll call her carrier for support to get it fixed.