A321 Captain reported a communication breakdown with Ground Control while figuring out an issue with the aft cargo compartment smoke warning light and ECAM message.

Narrative

After engine shutdown and completion of parking checklist at the arrival gate in ZZZZ; the Aft Cargo Bay Smoke Warning Light illuminated with ECAM warning. The aft cargo bay was still closed and the jet bridge was connected. However; the cabin door had not been opened nor [was] deplaning commenced; for unknown reason. The flight crew complied with the Aft Cargo Smoke Warning ECAM to include securing the cabin fans and activating the AGENT 1 discharge into the aft cargo bin. Unfortunately; flight crew could not communicate the smoke indication to the ground crew quick enough and the aft cargo bin was opened 1-2 minutes after the agent was discharged.Captain notified Tower of the aft cargo bay smoke indications and the fire trucks and personnel were needed at the aircraft. Inflight was notified and no obvious signs of smoke were evident in the cabin. The Captain could not contact Operations or the Ramp on the radio or intercom and gate personnel had still not opened the cabin door; so the Captain elected to open the cabin door. The First Officer went down to the ramp to notify ground personnel. He noted no obvious signs of fire or smoke at that time.The Captain was still unable to find gate agents [and] so ordered an orderly deplaning of the customers after notifying them that we had indications of smoke in the aft cargo bin but saw no signs at that time of smoke or fire in cabin or cargo bins. Captain tried to communicate to station firefighters of the situation and that the agent/Halon bottle had been emptied into the aft cargo bin. After deplaning and seeing no obvious signs of smoke or fire; the Captain called Dispatch/Maintenance on SATCOM to notify them of the agent discharge and the subsequent logbook write-up. Firefighting personnel indicated to the Captain that the situation was under control. Maintenance personnel arrived and Captain briefed them on the cargo smoke indication and logbook write-up. Captain notified inflight that the aircraft would have to be serviced before their return flight to ZZZ and that there would probably be a lengthy delay and that they should contact crew services. Flight crew departed the aircraft.