We propose a collaborative reputation-based voice spam filtering framework. Our approach uses the cumulative online duration of a VoIP user to derive his reputation value. And we leverage user feedback to mark unsolicited calls. For each unwanted call, our voice spam filter charges the caller a reputation point, and transfers this reputation point to the callee. To avoid VoIP users to manually label nuisance calls, our voice spam filter automatically marks all VoIP calls with short call durations as unsolicited. The preliminary simulation results show that our approach is effective to counter voice spam.