The network protocol basically controls the flow of traffic among links, i.e., how and when traffic leaving one link will flow onto another in order to get that traffic to its ultimate destination. IP is just one type of network protocol. The use of IP protocol has no bearing on whether the voice signal will be transmitted in a digital or analog form. Virtually all voice transmission within public carrier networks today are digital, as compared to analog transmission. Moreover, all such digital transmission is done using circuit switching, as compared with packet switching or more specifically, the IP protocol.
Digital transmission has some advantages to analog transmission.
Those advantages include digital's relative immunity to noise, and reduced maintenance costs. Without qualification, if one had to choose between a circuit switched network or packet switched network for carrying voice, circuit switched would be a better fit for the needs of voice communications. This has most to do with the fact that voice communications typically demands the continuous use of a voice grade channel. In contrast, most data communications tend to be bursty. However, voice communications is highly sensitive to delays or irregularities in the transmission of voice signal components.