Personal communications include conversations, letters, interviews, e-mails and telephone conversations.
• IEEE style states that you cite only published works, forthcoming published works, and unpublished materials available to scholars in a library, a depository, or an archive.
• For interviews or other "non-recoverable" information, no citation number is necessary.
This does not mean that an attempt to identify the author or the date of the communication is unnecessary, but that it needs to be done in the text itself:
"In a personal interview with Bill Gates on June 28 2006, he suggested that he would soon rule the world."
"In a letter to the author dated September 28 2018, Professor Mueller detailed his experiences with using this data collection software."