Hello @turtlak,
When you speak about UPC box, are you talking about the Horizon box or another type of box?
The Horizon box is not amplified and as such there is no volume to control on it. The RC will always control a TV or eventually an external amplifier.
Now if you have lets say a “Yamaha” amplifier, you need to program the code “3326” in your RC but with that you will most likely lose the possibility of switching on/off your TV set.
If you have recent and good quality devices (TV and Amp) you should have an ARC connection between the TV set and the amplifier and with that when you control the volume of your TV it should automatically control the volume on the amplifier through ARC.
If you have old or cheap devices then your best shot will be with a Logitech Harmony RC I believe or the good old fashioned method that every housewive love… the collection of RCs on the living room table…