Air carrier First Officer reported the failure of a main hydraulic system during descent.

Narrative

Descending through 20;000 ft we received an ECAM alert that the green hydraulic system was low which was verified on the indicator. I continued to fly while the Captain completed the steps on the ECAM. Once completed; the Captain [requested priority handling] because we would have to manually lower the gear and would need to be towed off the runway. We called the ramp numerous times to arrange a tow; but there was no answer. We were able to get tower to send someone over to notify the ramp. I continued to fly while the Captain opened the QRH and completed all steps and then briefed emergency gear extension. Once we were configured and ready for gear extension; the Captain took the controls and I used the QRH to complete the Emergency gear extension. With three gear down; I took the controls back and we made an uneventful landing on XXR. A tow tractor met us on the runway and towed us back to the line.Cause: Failure of the green hydraulic system. Suggestions: Only issue that was an avoidable safety concern was the lack of response from the ramp. This is not the first time I have been unable to reach the ramp. It has always been because of either a broken radio; or lack of personnel to have the ramp office monitored at all times. This is a safety concern that should be addressed and fixed.