As a slightly different take than JohnFx's answer yes, you can submit a Freedom of Information Act (FOIA) request with the various intelligence agencies to request copies of the data they have on you. However, odds are it will be redacted to a point of it being worth very little aside from an acknowledgment that an agency has collected on you. That is, of course, if they are even authorized to release anything at all.
To explain why that is, we have to go to the origin of authorization to collect on U.S. persons: Executive Order 12333. In essence, there are two legal means for intelligence agencies to collect on U.S. persons:
- If it is collected in the process of collecting on a non-U.S. persons target, e.g. a U.S. person called a non-U.S. person being collected on.
- If the collecting has been approved by a Foreign Intelligence Surveillance Court Act (FISA) court.
Both of these entail classified means, classified targets, and classified reasons, so odds are that any records collected on a U.S. person will not be permitted to be released in anywhere near complete, lest sources and methods be compromised. So all that being said, yes you most certainly can request a copy of collected phone calls, but no, most likely they will not be released to you.
I am sorry for your loss.
As full disclosure I work for an intelligence agency so my answer may reflect biases.