Voluntary response means that the subject selected themselves voluntarily for the survey.
Participants can be sent an invitation to voluntarily respond to a survey, and usually those respond more often who have a stronger opinion about the subject. This is why the sample is not representative of entire population.
Sending a link to participants (even randomly) to fill up an online survey.
Radio show that asks people to call in and show their opinion about a topic.
Optional surveys given to people walking by in shopping malls. (this will also introduce bias from convenient sampling)