Mobile VoIP is the term for carring voice calls over cellular mobile packet data services, such as VoIP over UMTS and VoIP over GPRS. Mobile VoIP brings dramatically reduced costs for mobile phone users who otherwise pay for many hundreds of minutes of traditional circuit-switched voice calls per month. In the UK, the mobile VoIP operator 3 supports free Skype calling over it's 3G UMTS network; however true Mobile VoIP uses standardised SIP-based signalling rather than Skype's proprietary mechanisms.
Unlicensed Mobile Access (UMA) is another implementation of Mobile VoIP, but using VoIP over Wi-Fi - heading towards Fixed Mobile Convergence (FMC).
How to implement Mobile VoIP
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