Remember I think they are innocent, so I'm still on the side with the angels.
Well, it's my understanding that they decided that Knox was the one who was going to call Kercher and Romanelli's English was not top notch anyway, to say the least.
I'm not saying it's decisive as a sing of guilt or anything, but I would have called again as soon as I got to voicemail, if I really, really wanted to get hold of someone. I can see how this maybe could add to suspicions already held by the police. My interpretation is that Knox and Sollecito at this point had begun to realize something was very much amiss with Kercher. They had discussed what Knox had seen over their meal. But if they were innocent, which I think they were, they didn't yet think of murder. I think you don't jump to that conclusion easily, because the mind halts at thinking the worst.
What about the turned off phone that rang? Is that possible or is Knox's recollection wrong?