United Jet seems to have ceased operations.
I found the following tie-ups all from several years back but there looks to be no correlation between callsign and tail number.
Jetcorp 100 N188TG Gates Learjet 60 Sep-06 UEJ100
Jetcorp 150 N660AS Gates Learjet 60 Dec-03 UEJ150
Jetcorp 300 N273MG Gates Learjet 55 Sep-04 UEJ300
Jetcorp 350 N122LX Gates Learjet 55 Dec-06 UEJ350
Jetcorp 600 N550HG Gates Learjet 55 Feb-04 UEJ600
Jetcorp 700 N574DA Gates Learjet 60 Feb-04 UEJ700
Jetcorp 850 N730M Gates Learjet 60 Aug-07 UEJ850
So your LJ55 could have been any one of three (or more).