Hypothetically speaking, if Nicola was abducted it would suit the abductor to make it look as though she had slipped and fallen into the river. With the phone on the bench it certainly looks like she had left the phone momentarily, as she stayed logged in. It would also allow some leeway in terms of time. If the phone was thrown into the river the water might kill the call early, which could flag someone's attention (why she left the call). I don't want to get my information wrong, but did I read somewhere that Paul is sometimes logged into those calls too? If I'm right, then it might prompt him to call her.
I agree, the wording 'moved to the bench area' is interesting. Someone moved the phone there, and it may not have been Nicola.