British engineer Saad al Hilli, from Claygate in Surrey, recorded all of his calls, which are in the process of being analysed.
Mr al Hilli and his wife Ikbal were gunned down on September 5 last year as they holidayed near Annecy, in south east France.
Mrs al Hilli's mother Suhaila al-Allaf and local man Sylvain Mollier also died, and the al Hillis' daughter Zainab was shot and beaten, but survived.
Annecy prosecutor Eric Maillaud said: "The victim recorded all his telephone conversations. We therefore have some very precise details."
Mr al Hilli's brother Zaid, from Chessington, Surrey, is due to answer police bail next month after he was arrested on suspicion of conspiracy to murder amid claims he doctored documents to ensure he inherited the family estate. Mr Maillaud said the inheritance dispute remained a central line of inquiry. They are looking into the brothers' father's property in Baghdad.
French and British investigators held a joint press conference in Annecy yesterday to give an update on the investigation, just over a year since the family was killed.
Mr Maillaud said: "We have made great advances."
The horrific murder scene was discovered by cyclist Brett Martin who found Mr Al Hilli, 50, his 47-year-old dentist wife and her elderly mother blasted to death in their BMW.
Zaid al Hilli denied any knowledge of why his brother was killed when approached by Sky News.