Some Verizon employees may have had unauthorised access to their President-elect's personal mobile phone account.
Details that the employees could have viewed, according to experts, include the numbers a customer had called, how long conversations with those people were and when he called them.
The phone account is no longer active and an aide told Reuters that Obama's voice-mail messages and emails were not breached in the incident.
"We were notified yesterday that employees had accessed the records of an old cell phone no longer in use", the Obama aide said. "No voice or e-mails were listened to or read."
But Verizon has taken immediate action and has put the employees involved on immediate leave.
In a statement, Verizon Wireless president and chief exec Lowell McAdam also apologised to Obama in a statement, and added that those who accessed the account "improperly" will face disciplinary action.