Yes, I would suggest that when a potential focus group participant returns your call and gets your voicemail, you should add the following: "...If we do not respond to your call within the next (minutes, hour/hours), then we have already filled spaces for this opportunity. Thanks again for taking the time to complete the online survey and please check our site regularly for future opportunities." Simple, kind gestures like this go a long way so people are still left with a positive experience with you. And hopefully you won't be bombarded with people repeatedly calling back.