Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness; Communication Breakdown
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Environment - Non Weather Related; Human Factors; Procedure
Upon contacting Departure Control after takeoff; we were cleared to 16;000 feet.  We were vectored west as usual. Climbing through about 14;500 feet; ATC told us to descend back to 14;000 feet saying he had given us a clearance to stop at 14;000 feet.  We complied. We were never given a clearance to stop at 14;000 feet. The traffic passed a little bit later and we continued on without incident. The frequency was very congested with people stepping all over each other. If the Controller did give us a clearance to stop at 14;000 feet; we never heard; nor acknowledged it.
Believe we were already handed off to ZZZ Center. On initial contact with them they cleared us to climb and maintain 16;000 feet MSL.  Passing 14;500 feet; the Controller asked us to return to 14;000 feet MSL as he asked us to.  The Pilot Monitoring stated that we never received that clearance and were climbing to 16;000 feet MSL as previously cleared.  The Controller then stated that we had conflicting traffic and must go down to 14;000 feet MSL.  I immediately leveled off the aircraft on the initial call and then proceeded to return to 14;000 feet MSL once the situation was clarified.  The aircraft reached 15;100 feet MSL during the transition back to a descent.  I am confident we never received the call to stop the climb at 14;000 feet and am sure we never responded to such a call.  The radios were busy and ATCs initial call to level off may have been blocked by other transmissions.The Crew reacted appropriately with the information we had received during the situation. It has become apparent that ATC Personnel are working multiple frequencies since the reduction in flying due to COVID.  In my opinion; the flying is returning faster than the ATC manning and causing some increased ATC workload and some communication errors.
Air carrier flight crew reported ATC requested they descend back to their cleared altitude of 14;000 during departure.  The flight crew stated they never heard ATC cleared them to 14;000 feet and cited frequency congestion as a contributing factor.