Until a call is answered (in most cases in the PSTN world) the ringback tone is provided by the originating, or terminating carrier depending on how SS7 is set for the carriers involved. Some open an audio path as soon as the terminating line is proven to be idle, some wait until answer supervision. When you are dealing with a VoIP call from end to end, it can get even trickier. It may, or may not be 3CX's ringback tone that customers are hearing before the call is answered, I suspect that it is not. 3CX seems to be capable of using one set of "regional tones" at a time and a change to this affects a number of things for all users.
If a UK caller dials a number in North America would they not expect to reach a North American ring-back tone in the same way that if I call a UK number I expect a UK ringback tone.
Now, if you are trying to "fool" the UK callers into thinking they have reached someone else in the UK, because they dialled a UK number you may be out of luck. I have been in the UK and called a number of businesses (PBX's) and encountered North American tones, so not everyone there sticks to the traditional UK tone plan