How do you figure out who is calling you with blocked caller ID?

[discostew]

Some asshat has a robo caller machine and is making calls to a little old lady I know. Her phone just shows a blank screen when the calls come in. It calls her every 15 min.
So how do you figure out who is making those calls?

 

[JUKE179r]

Does she have a smart phone or a land line?
There is a way to block it even though no caller ID. https://macreports.com/iphone-block-no-caller-id-calls/

 

[JohnnoK]

I don't know if it will work in the US, but there is an App called Truecaller that filters a lot of shit on my wife's phone.

 

[CORover]

Try hooking up a fax machine to that line for a while and see if they go away.