Nowadays Voice over Internet Protocol (VoIP) is becoming an evolutionary technology in telecommunications. In this paper the study is focused on the resources that are allocated for VoIP calls. Resource allocation in a VoIP system and resource degradation when new demands for resources arrive at the system are modeled. To counteract resource degradation and improve availability and service reliability, we propose to perform software rejuvenation.. Moreover, the rate of resource allocation in such a system can be importantly affected by the time that an amount of resources is allocated in order to serve VoIP calls. Hence, we model software rejuvenation in a VoIP system with a semi-Markov process in order to capture the effects of time spent at resource degraded states of the system. Through the stochastic analysis of the system an optimal rejuvenation policy that maximizes system?s availability is proposed and furthermore the corresponding reliability levels in means of Mean Time To Failure are derived.