This is what happened - we know these facts.
Child A was alone in the house, had been specifically told to ring her mum if anything went wrong or she was anxious. She was previously told to ring her dad or a neighbour if mum ws unavailable, but was not reminded of this previous instruction.
We're not specifically told this (all the messages and calls on the mum's phone could have been from other people) but it seems that Child A tried to ring mum. We know that mum's phone was switched off.
What I think happened from this point.
I don't why the child made the initial phone call - it could even have been to ask where the Nutella was. But in her situation, if my mum had told me that they would be contactable and then they weren't, I would panic. I'd panic as an adult, let alone as a 12 year old who may already have been scared of something which prompted the initial phone call.
We don't actually know if the child rang her father, or even if the father was actually on standby. Given the fact that mum forgot to remind the child of the arrangement, I would suspect that dad hadn't been warned to be available. Perhaps he wasn't available either - I do hope I am wrong about this. I can also think of lots of reasons Child A might not want to phone her father - scared about passing on bad news, scared that mum might be somewhere she shouldn't be and a call would get mum into trouble.... all sorts of scenarios.
Child A rang her friend, and reported that despite several tries, she couldn't contact her mum. (If I were Child A, I'd be pretty freaked out by now. My mum, who promised she would be contactable, isn't answering her phone. She must have had an accident. And I am alone in a scary house.)
We all (if we have read and understood the post) know what happened from here on. I think it's unlikely that Parent of Friend made no attempt to contact mum - most likely thing is that she personally witnessed calls to mum by Child A going direct to voicemail. She may have left messages herself - we don't know who all these multiple messages were from.
And the cherry on the cake - mum accuses Child A of being manipulative. YABVVU, OP! (And I also have my suspicions that the time out of contact was rather longer than an hour. Again, I hope I am wrong.)