The pilot in command was a Training Captain and he occupied the right seat, operating the radios and navigation equipment. The other pilot was part way through his training for Captaincy on type and was flying the aircraft. After take off the crew were initially given radar headings by Melbourne Departures Control. At 0706 hours they were instructed to resume their own navigation. Some four minutes later while the Captain was making his normal public address announcement to the passengers concerning seat belts the Departures controller asked the crew to "-- just confirm tracking direct ---." The crew reaction to this was to ask themselves if they had made an error. The pilot in the right seat made a careful check of the navigation equipment, confirmed that all was in order and then advised the Controller of this. While nothing was found to be wrong this unexpected query from Departures Control remained in the minds of the crew. At 0710 the crew were instructed to change to frequency 128.5 MHz and call Melbourne Control. This instruction was acknowledged. No call was received from the aircraft crew and further calls directed to them on various frequencies did not obtain a response. The aircraft was tracked on radar continuously and when it was approximately 150 miles from Brisbane the crew contacted Brisbane Air Traffic Control and asked for a clearance to descend. Communications were normal for the remainder of the flight. Monitoring of the communications recording for the departure from Melbourne showed that the instruction to call on frequency 128.5 MHz tapered off so that the "five" was very difficult to hear. Advice from the crew was that they had tuned their equipment to 128.4 MHz. This was one of several Sydney Air Traffic Control frequencies and the crew heard periodic radio transmissions to and from other airraft, which indicated to them that all was normal. When the crew changed to 128.4 MHz they called on that frequency and thought they obtained a response. The aircraft was operating under full radar coverage which meant that the crew were not required to make any position reports to Air Traffic Control. The procedure under such an environment is for the crew to be told to change from one frequency to another. As this was a training flight considerable in flight time was devoted to training tasks, in particular cross checking of radio navigation aids against the aircraft's main navigation system. Attempts were also made to contact the crew on the emergency frequency of 121.5 MHz, without success. Monitoring of the Sydney frequency 128.4MHz did not reveal any calls from the aircraft, which was to be expected as it would have been beyond the range for two way communications with the aircraft at the time. However, it is possible that the crew heard part of a transmission from another enroute aircraft communicating with Sydney on this frequency. No calls were made to the aircraft on 128.4 MHz, which was not one of the frequencies that would have been used on that flight.