Report from Vietnam: I received scam calls pretty often before. It Vietnam trash SIMs were fairly common, and it bothered people so much that the goverment has to force every mobile company to "standardize" their consumer's data (i.e. fixing the incorrect data in trash SIMs). I have not received any scam calls since doing it.
Basically SIMs owned by previous owners that are brimmed with scam messages and calls. They are much cheaper than new legitimate SIMs so many people choose them. Sometimes people buy trash SIMs that has "good" or "lucky" numbers (something like 999 999 9999)
