I think they said phone was off because kids message about pizza came through when phone was switched back on. That's why she asked Nick where he'd been because she had assumed he was home all evening.
You're mixing up two things.
She got the message whenever she picked up her phone later (we don't know when that was, we don't know how long she stayed out, only that she turned up for work the next day late), nothing to do with it being switched on again, just that she got hold of it again so got the vm. She may well have left it on silent, mine nearly always is - then no risk of anyone else answering if it was also face down.
AC12 found the message later, after they sequestered her phone (it being a work one) and after she had told them she was home all night - they then said 'hang on, if she was home, why were the kids calling her?' and Kate said 'her phone was home but she wasn't' - it didn't just 'come through' because they turned it on, Roz had heard it before that. Then Steve went off to ask Nick where Roz was that night.
The only thing this helps with is whether she went out intending to attack Ifield.