mobile VoIP
Mobile VoIP refers to mobile handsets that have been integrated into an existing VoIP, or Internet telephony, network, typically via a Wi-Fi or 3G/4G cellular network that the mobile device is connected to.
In most cases, mobile phone users will need to download a service provider's software application to their smartphone before being able to make mobile VoIP calls from that phone. Once properly configured, mobile VoIP calls can be made anywhere the user has access to a Wi-Fi or 3G/4G network.
See The Difference Between VoIP and PSTN Systems in the Did You Know... section of Webopedia.
Also see the VoIP Meets Wi-Fi article in the Did You Know? section of Webopedia.


