A320
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Human Factors
Captain was PF (Pilot Flying) and I was PM (Pilot Monitoring.) On the Eaglz2 RNAV arrival ATC gave us a heading off of the arrival and to descend and maintain 4;000 feet. I think this was when we were at about 8;000 feet. I activated the approach and sequenced it for the visual backed up ILS 26R. After that as we were descending I was looking out the window at the terrain and houses but did not think we were in danger but just how low we were as we crossed over that area. To me this was not unusual because once crossing the terrain you are usually high on the approach. So I didn't think it was off. I go to ONT maybe once every 9-12 months. Both the Captain and I had our terrain map up on the MFD.At about 6;400-6;500 feet ATC gave us a terrain alert and to climb to 8;000. I noticed the Captain did not react so I did and climbed the aircraft. On the way up I responded to ATC that we were in a climb to 8;000. ATC said I must have taken another aircraft's radio [call]. I do not agree; but I am human and do make mistakes so it is possible but I still don't think I did in this case. There was no evidence of stepping on anyone on the radio. The Captain and I both agree we heard our flight number and to fly present heading followed by a descent to 4;000 feet. I read back as such. Later we briefly discussed it with ATC. He claimed it was for another aircraft on the radio. I still believe ATC did tell us 4;000 feet. Either way I realize in the future when around terrain to look at the highest terrain altitude readout on my MFD map and reference that to where it is compared to what ATC gives us to be even more aware. We never got any GPWS alerts at all.
A320 First Officer reported receiving a low altitude alert from ATC on descent into ONT after apparently taking a descent clearance meant for another aircraft.