Small Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine; Other EA50;HA420;F9F-2 Typed
Flight Crew Last 90 Days 20; Flight Crew Total 4300; Flight Crew Type 145
Communication Breakdown; Troubleshooting; Confusion
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Environment - Non Weather Related; Human Factors; Staffing
I filed IFR flight plan. The flight was uneventful. I was cleared by ZZZ Center to descend to 10;000 feet MSL. I confirmed that clearance. I descended and was level for a minute or so when the Controller said to climb back up to 11;000 feet which was the altitude to which Controller cleared me. That was incorrect and would have been the wrong altitude for that direction of flight. The Controller admitted on the radio [to] working six frequencies at once. There was no traffic conflict. I think it is unfortunate that the Controller was so busy and the transmissions revealed Controller was very new on the job but the Supervisor should have helped out. Controller cleared me to 10;000 feet about a minute after Controller told me to climb up to 11;000 feet. Controller was clearly confused.
Corporate Captain reported the Controller was working 6 frequencies at once and issued an incorrect altitude clearance.