The report was released under the Official Information Act and shows numerous failures by Corrections staff allowed the man to make the calls.
The report said the phone number was lodged on an official approved list even though no screening process had taken place.
Police learned on October 28 that the prisoner had been making phone calls from the prison and had possibly spoken to the victim at that phone number. Police told the inquiry that they immediately asked for access to the phone number to be blocked while they checked voice recordings.
After listening, officers told prison staff they had to stop the prisoner from using that phone number. The request was passed through three Corrections staff without any action being taken.
Three further paperwork failures allowed the prisoner to continue calling. Reports detailing the police request were written but not sent. Then reports were sent without explaining the police request for the number to be blocked.
On March 21, after the Herald on Sunday revealed the prisoner had contacted his victim, the phone number was finally blocked.
The investigation into the breaches found basic systems failed. Phone numbers nominated by prisoners should be checked by prison officers. It emerged the numbers were being called by the prison receptionist, who did not follow the checking processes.
In a statement, Corrections chief custodial adviser Eric Fairbairn said staff had been held "accountable" but offered no detail. He said official processes were not followed.