I think it was kind of mimicking when Ronda called Michelle I think her name was and left that voice message. I think that's the episode your talking about.

One of the bus crash victim's friends, Rhonda had a voice message off her mate who died. It had her talking for a few seconds then you heard an explosion and then you heard the bus crashing off down the cliff.

Veronica is being haunted by the people who were on the bus on 'I Am God'. The mobile ringing is just saying that the bus is ready to crash.