A former FSDO and I were having dinner the other night and we got into this discussion of investigations. I asked him exactly how he would handle a phone call inviting him to have a discussion with an investigator. He said that he would make up some excuse as to why he could not speak at that very moment (even if the *excuse* was because he needed to review the facts in his head) and then immediately call an Aviation Attorney who's first act would probably be to draft a letter to the FAA directing them to make further inquiries through him.
Sounded like pretty good advice.
USMCFLYR