The bench where her phone was found has been left in situ. If there was any doubt whatsoever that the police had if she came to harm by someone else, then that bench would have been removed for a forensic analysis. There must be a reason known to the police that she left that bench of her own choice, but it is not being made public.
Any detail regarding this work call has not been released. It seems odd that it was her last known contact with anybody, yet no detail at all is being disclosed. I can't see any other reason how the police could rule out a third party actor, other than what happened on that call. But they clearly are.