If foul play was involved, then the phone was probably tossed.
I do think that the apparent ringing of the phone months later can be explained by a busy tower bumping the call to a different tower. That happened right after the beginning of the semester, and towers near the campus would have been very, very busy indeed.
I still consider an accidental death and concealment in the construction area the leading theory, but a death or body dump in the river is the runner up. Many drowning victims are never found. If a drowning victim has a low percentage of body fat, as Brian appeared to, then it is more likely that the body will not rise to the surface.