Is using the word "halo" (allô) forbidden, and if so, what is the proof of its prohibition?

The word "Hello" (آلو) used at the beginning of phone calls is not forbidden (haram), for the default rule concerning things is permissibility unless there is evidence to the contrary. The Sunnah is to begin with "Salam" (peace) before speaking. As for one who does not begin with "Salam," it is legislated not to respond to them, as there is a Hadith to that effect. However, beginning with something other than "Salam" is not forbidden (haram) because "Salam" is not originally obligatory.