Jean, while businesses are required by law to buy the Do not call lists, there are plenty of unscrupulous ones out there who don't. When you get a no-answer, it is likely because the "agent" on the other end has not yet answered when your phone picked up.
I don't usually answer "suspicious" calls, but sometimes I do so by mistake. When that happens, I ask the caller if they purchased the required do not call list. That question usually gets me hung up on by them!!

The next thing I do is go online and report it on the do not call registry.