I count three calls, the first one on June 5, and two more which evidently were not recorded on June 7 at approximately 3:30 PM heard by Jannelle, if I read the narrative correctly.
While we can logically conclude the two calls of June 7 were from the same individual, we can't also conclude the first erased call was from the same person.
If there were two different people, the one from June 5 and the two from June 7, then the erased call could have been critical for voice identification purposes. Obviously the FBI would have been ideally positioned to potentially identify this person with their sophisticated voice analysis technology.
The question I would ask is even if the same person placed all three calls why would the Levitt residence be targeted? If such calls were being placed all over the area, it would appear this individual must have spent most of his time making such calls and is obviously mentally ill and not likely a viable suspect. But because we don't know the identity of the first caller or any way to compare the calls we can't say whether there are one or two individuals.
A thought: We do not know that the suspect who was making these phone calls "all over the area" is the same person who made the two phone calls on June 7. Is it not conceivable that the abductor(s) were placing random telephone calls to the residence to determine if anyone had happened onto the crime scene? While he/they would not have wanted to reveal themselves by revisiting the crime scene that day it would be in their interest to see if some commotion was developing there in order to adjust their activities to evade detection; possibly moving the women if they were still alive. An impersonal phone call tells them that there are people on the scene and the "cat is out of the bag." The vulgar comments might simply have been an afterthought. Obviously, the caller wouldn't have come out and asked if the cops were on the scene yet. It could have been yet another "ruse" to gather information if the receiving party would blurt out that "give us a break, we have three women missing." Do we know what Jannelle may have said to this person?
A subsequent phone call to the McCall residence was of a woman stating the "old woman was dead" and the two girls dead as well. But that was AFTER the news reports had come out. The phone calls to the Levitt home were well before this news item hit the airwaves and newspaper. That person was identified and had nothing to do with the abductions. I don't recall what the outcome of the incident came to.
"Never answer an anonymous letter"
"I didn't really say everything I said"