A B767 Captain allowed the Ramp Agent pushing the aircraft to use hand signals because the tug's intercom failed. After the push was complete the Agent failed to advise the Captain to set the brakes and the aircraft began moving on its own.

Narrative

As we were being pushed back; the pushback driver stopped the push and called me to say his cord for communication broke and could not talk to me from the cab of the tow so he asked if we could use hand signals and I agreed to do so. He went back to the cab and continued towing us out to the top of the alley and stopped. We waited for hand signals but nobody appeared to give the set brakes signal so we waited. The aircraft began moving forward again so we thought we were under tow again until I saw the tow out of the corner of my eye to the left. We then realized we were not connected and were rolling forward. Both First Officer and I hit the brakes and came to a stop. I looked out and saw a guide man give me a signal for release. I gave him a shoulder shrug to say what was going on. He waved me off. Since we did not feel any damage to the aircraft and it seemed no one got hurt on the ground we continued on. A Flight Attendant did get hurt when we came to an abrupt stop but only minor injury. The rest of the flight was uneventful.