Ostarbeiter or eastern workers made up 3-5.5 million of the German workforce, some report numbers as high as 8.5 million.
They consisted of Ukranians, Belarusians, Russians, Tatars, and ethic Poles from USSR territories.
They initially did volunnteer in early 1942, lured by promises of plentiful food, good working conditions, and freedom. But when the word filtered back from Germany that the promises were lies, Germany had to fill their worker quotas with mass roundups.
The east workers were required to wear a blue tag with OST on their outer clothes at all times, and were kept in camps under armed guard when off work. That doesn't quite sound like the way you would normally treat "volunteers"
Even though they were taking 40,000 people a month from the Ukrain alone, Speer was constantly complaining about a shrinking " volunteer " workforce, so that speaks as to their treatment.
Though some may have "volunteered" for Flak crews, it's not hard to reason out the why of that.
After the war the former OST workers, all of them tainted by the initial volunteers, were all treated as traitors by the USSR. At the best they were sent to workcamps in Siberia.