Medium Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Procedure; Environment - Non Weather Related
Due to low ceilings at LGA; a runway change was issued; therefore sending many aircraft; including ours into holding patterns. The radios were congested. We were on the KORRY 4 arrival and were holding at HOLEY for about 15-20 minutes.After a couple vectors out of the hold; at an altitude of 13;000' MSL (not 100% sure of altitude); we were then given a clearance of what I (Pilot Monitoring) interpreted as direct to BRAND and descend via the KORRY 4 arrival. I read back 'direct BRAND and descend via KORRY 4'. There was no response from ATC that I read back the instruction incorrectly. The Captain as Pilot Flying then preceded to descend via the KORRY 4. Once we reached approximately 11;500' MSL; ATC told us that we were not given clearance to 'descend via'. ATC then gave us an altitude of 11;000' MSL.Apparently there was confusion as to what ATC instructed us. I did not feel uncertain about the clearance; especially since my read back was not questioned by ATC; otherwise; I would have inquired. I'm not sure whether I or ATC communicated incorrectly. Regardless; I would say the most contributing factor or threat was radio congestion.
Pilot reported having an altitude excursion; citing radio congestion and nonresponse or correction from ATC as causalities.