The 3rd Generation Partnership Project (3GPP) opted to use the SIP protocol to provide services a number of years ago. This not only allows for VoIP calls but also for use of other features such as presence and instant messaging.
VoIP abilities are also now present in many mixed mode cellular and WiFi handsets. Here is one example of such a handset. So, the answer is that VoIP is implemented in cellular phone systems today.
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