Flight Crew Air Transport Pilot (ATP)
Confusion; Communication Breakdown; Distraction
ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert
Human Factors; Environment - Non Weather Related
Upon initial contact with Approach we were told to expect the visual or RNAV Runway XXR.  At approximately mid field right downwind we were instructed by Approach Control to contact Tower.  Upon contact with Tower we were cleared to land on Runway XXR.  We continued our descent and approach.   A little while later; Tower instructed us to return to approach.  Approach Control advised us of a possible pilot deviation as we were not cleared for an approach.  We were given a base turn and asked if the airport was in sight.  We were then cleared for a visual approach and again instructed to contact the Tower.  In hindsight; the instruction to contact the Tower came at an unusual time; mid field left downwind.  But when cleared to land by the Tower; I thought nothing more of it.  It was the instruction to return to approach and the subsequent notification of a potential pilot deviation that causes me to question the initial instruction.Pay closer attention when instructions are given in a non-normal order/environment.
Approach Control switched us to Tower. Checking in with Tower we were cleared to land [Runway] XXR. So we descended prior to ZZZZZ (on the arrival is 210 knots at 11;000 feet.) After Tower cleared us to land he sent us back to Approach frequency where we were told we needed to get a phone number for a possible violation. After we received landing clearance we did descend prior to ZZZZZ to approximately 10;000 feet. With COVID procedures in effect maybe we should have questioned the Tower about the frequency change; but with so many towers at 1/2 staff and/or closed I do not know what all the different operating procedures are from airport to airport. Being cleared to land on downwind maybe unusual but not unheard of.Closer monitoring of flight number on the radio but I am absolutely sure I heard him switch our flight over to Tower. I really didn't have any clue something was wrong until the Controller told us of possible violation.
Air carrier flight crew reported they were informed by ATC of a possible altitude deviation.