Real-time VoIP communication demands strong QoS assurance for its efficient performance. Ensuring high quality of VoIP transmission in spectrally congested scenarios requires deployment of VoIP applications in intelligent Cognitive Radio Networks (CRN) that aim to increase spectrum utilization through its opportunistic mode of communication. However, complexities inherent in unmodified CRN make it unworthy of hosting VoIP applications. The objective of this work is to successfully implement VoIP service in CRN by optimal selection of transmission time so that adequate QoS is ensured for VoIP users. Considering imperfect spectrum sensing, an algorithm is proposed to select the most efficient transmission duration for VoIP communication by taking into account relevant parameters in CRN. Mathematical analysis followed by simulation results provides clear testimony to the fact that VoIP call quality is enhanced after implementing the proposed algorithm.