If the bus doesn't show up? This must be public buses and not yellow buses. Call your council and tell them to fix the transit system. Meanwhile a child can walk home and get a ride, or show up to the stop earlier.
She can call from where she goes after school, no? Or she can go home first, or make plans a day ahead.
She can learn the consequences of not packing her school bags properly and keep a checklist to review each morning. The only time I recall in 12 years of primary schooling, where I had to get something from home after forgetting, was a prop for a demonstration in front of the class. I remember because it was the only time it mattered.
I am not familiar with your specifics, I don't mean to be personal. I don't have kids. But I am young enough to remember being one - I am also addicted to my phone, and I know how convenient it is to not plan anything and to instantly communicate with everyone. I am unconvinced that children in a controlled, supervised environment need a phone.
I say all this with humility because I haven't dealt directly with kids or the public school system in a long time.
...finally, if I did agree and say "yeah they should at least be allowed to have a phone on their way", it should absolutely be banned in the classroom. But what to do when the children inevitably break that rule? It doesn't sound like you would support them confiscating it, and it's a logistical quagmire to do so anyway.