VoIP (Voice over Internet Protocol) telephony has already been widely used in fixed broadband networks and, with its low price, has posed a significant challenge to traditional telecommunications. Following fixed-line telephones, mobile voice communication has become the next market that VoIP network telephony aims to capture. Just as traditional mobile phones had a major advantage over traditional fixed-line voice services, mobile VoIP telephony can provide greater flexibility while drastically reducing costs, allowing users to enjoy VoIP's low rates anytime and anywhere. However, compared to VoIP network telephony based on fixed broadband networks, mobile VoIP has different characteristics and implementation methods. Depending on the differences in implementation methods, mobile VoIP can be divided into several main categories.