Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Communication Breakdown; Human-Machine Interface; Troubleshooting
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Environment - Non Weather Related; Human Factors
We were at cruise [and] ZZZ Center told us to descend via ZZZZZ landing north.  We read it back and continued.  After the ZZZZZ1 turn they asked if we were on the ZZZZZ1 or ZZZZZ2 [transition].  We said the ZZZZZ1 [transition] and the Controller said it looked [like] we were cleared the ZZZZZ2 transition.  Controller said no problem and we continued to descend via while reviewing the fixes again and discussing how it could have happened.  After a few minutes there was a different Controller on the frequency and we could hear him talking to other aircraft.  So we queried him a few times to no avail.  I then input the approach frequency from the IAP and contacted them there since we had not heard a hand off.  They said they were trying us on guard for the last few minutes.  Since we were still at 9;000 instead of diving and spoilers etc.; we elected a box vector for the descent.  The flight continued with no further incident. [Cause was] improper FMS entry.  Still not sure when it changed as we briefed it properly; but we were given the ZZZZZ4 by ZZZ Tower. After push and an FMS change was made at which point the ZZZZZ1 transition could have been selected instead of ZZZZZ2.  First trip back after more than 2 months off and forgot to input guard into the standby like I do every other flight.  One of the small but beneficial habits that begin to fade without repetition and use.
Air carrier Captain reported flying the wrong transition on a STAR due to a possible FMS entry error.