Almost guarantee your problem is with the UCT Brush. Check:

1. Brush must be of the Insulated Butt style (Contact segment must not wrap around the hinge end.
2. Brush must not touch the contact segment on adjacent floor bars simultaneously. Understand your reluctance to uncouple selector, but you won't get into any trouble as long as you mark position of selector carefully and re-couple at same point. The "0" mark on the dial should be lined up with the pointer when the elevator is at one of the middle most floors. That is the best place to uncouple. Simply remove the two 0.375" bolts in the hub. Crank the selector up / down and make absolutely sure that the UCT contact segment never makes contact on two adjacent floor bars at any given time. The insulated butt should lift the brush up before the contact segment makes contact on the next floor. Check brush type. Should be same style as DCT, UHT, and DHT. If the brush makes contact with adjacent floor bar segments, when controller issues cancellation signal, both call will be cancelled.
Good Luck.