A B767 captain reports being confused by a Customer Service Supervisor at an international station who is using the pushback crew's headset to request brake release time. The brakes are released prior to being officially cleared to do so by the pushback crew.

Narrative

The pushback crew reported ready over the flight interphone. This station requires flights to be 'fully ready' before a route clearance can be obtained from Air Traffic Control (ATC). So we proceeded to obtain our route clearance only to be interrupted by another voice on the flight interphone with a lengthy; non-standard transmission I did not understand. I responded 'standby.' My instruction generated another lengthy; non-standard transmission that I ignored to concentrate on our ATC interactions; including both the route clearance and a pushback clearance. Then I returned my attention to the flight interphone where I ultimately understood the woman on the headset wanted pushback clearance and notification of the official brake release time. I cleared her to initiate push and reported the brake release time as requested. Shortly thereafter the original voice requested permission to initiate pushback. There is no room for confusion during pushback let alone confusion over who is speaking on the flight interphone. I obtained absolute clarification that this original voice was indeed representing the pushback crew. I was told the other voice was a Customer Service Supervisor who had been eager for an on time departure. I had no way to know I had released the aircraft parking brakes based on instructions from a customer service representative rather than the authorized push crew. The unauthorized brake release was inadvertent on my part. In spite of the interruptions there were no misunderstandings regarding ATC clearances and no associated violations. I told the pushback crew that I never wanted them to yield the headset to a customer service supervisor again. We returned to professional status and resumed the pushback. One way I might avoid such a circumstance is to refuse to release the aircraft brakes if the headset operator deviates from the standard dialog. However; push back crew at international stations such as this serve multiple airlines and seldom conform to the dialog specified in our manuals. To refuse brake release due to dialog deviations would mean the vast majority of flights would not operate.