This telephone number first aired during Season 4 Episode 2 ("Confirmed Dead") after being a clue in Chapter 1 of the Find 815 Alternate Reality Game.
Here is a transcription of the message that is heard as of airdate (Feb 07, 2008):
"Thank-you for calling the Oceanic Airlines Flight 815 Hotline. Information is updated as the investigation proceeds. Investigations are thorough and as such make take a period of time to complete. At this time we do do not have any additional information regarding crashed Flight 815. If you are a family member or relative of passenger on Oceanic Flight 815 - We are sorry for your loss. Detailed information is given during the daily family briefings. This briefing updates families on the progress of the investigation, and allows for questions to be asked of the medical examinor or coroner and other parties connected to this investigation. Please contact your accident coordinator or call back later for more information."