So far, I think we only have the following leads:
It was a male caller and Frank didn't know the voice.
Personally, I have a few suspects there:
1. Becca. She is extremely clever and cunning. There's no way Shannon could have hidden that from her. Also, she wasn't surprised when Frank told her who her mother was having the affair with. She does say something along the lines that she suspected, but I suspect she had more than that. Even if she didn't make the call herself, she still would have had a friend make the call.
2. Someone in Jessica's family, namely the father. Possibly he had a guilty conscience about how he and his wife treated her. Maybe he kept an open eye on Abby and Frank from a distance after hearing that Frank married his daughter's adversary to avoid a drama like Shannon vs Jessica happening again. What argues against this is that Frank might have recognized the voice. However, Jessica and Frank had no contact with Jessica's family for many years. So enough time for the voice to change a bit and a phone can reinforce that.
3. this party girl's family lawyers. As we know the family, primarily the brothers and the mother, had guilt because their daughter/sister was the one who ran over Jessica. We likewise know that they kept an eye on Frank and Abby. What argues against this is that the topic of divorce came up and Abby was pretty composed, at least more so than Frank. So it would have made perfect sense to bring that up to convince him that you're on his side.
4. someone we don't know yet, but may yet get to know.
To be ruled out:
1. Abby's father. I don't know if he knew Abby's father's voice by then, but he probably would have recognized it after his phone call at the latest.
2. Lilly's father. To rub it in Frank's face. What speaks for it: Unlike Shannon, he wasn't really surprised to be caught by Frank. Considering that he just left his wife and daughter, it's also just an indication that he's a callous asshole.
-----
The private investigators are also an interesting thought. The only question then is: Did they not tell the lawyers that they had informed Frank? Even though they may have exceeded their authority, the lawyers could have used that to convince Frank that they were on his side.
Whoever did this, I hope it gets cleared up for us and for Frank.