we need to boycott companies which cannot prove that they don't use forced labour
I also remember a documentary when TWA ( way back ) used prisoners for call centers.
When people say there is no ethical consumption under capitalism, this is what they mean. No one's hands are clean here. It's a problem something as simple as boycotting won't fix, because you would have to boycott everything - the entire system runs on blood and violence, top to bottom.
But I can easily prove that Bigfoot does not exist within my coffee mug, or that a particular box I have which contains 5 pennies contains no silver dollars, or that the list of suppliers with whom I directly do business does not include "Missouri Correctional Services LLC" (and I think the spirit of GP's suggestion is boycotting companies that directly do business with prison labour, not people who transitively do business with it at a 25 company remove, or something)