A client device for a seamless handover of a transmission of a data application, comprising: a first signal transmitter configured to transmit over a first wireless network; a second signal transmitter configured to transmit over a second wireless network; a storage device including the data application and a Voice over Internet Protocol handover proxy ; wherein the first and the second wireless networks are different types of wireless networks, wherein the first wireless network comprises a wireless local area network, a wireless wide area network, or Worldwide Interoperability for Microwave Access, wherein the second wireless network comprises wireless local area network, a wireless wide area network, or Worldwide Interoperability for Microwave Access, but is not the same as the first wireless network; and wherein the Voice over Internet Protocol handover proxy is configured to: relay the data application on the client device over an active connection to a proxy server and a second client device ; transmit media of the data application over the active connection; identify a seamless vertical handover event; maintain the active connection over a first wireless network to the proxy server and the second client device ; initiate Session Initiation Protocol re-invite messages to the proxy server while switching the active connection from the first wireless network to the second wireless network; and maintain the active connection and transmission of media of the data application over the second wireless network. a first signal transmitter configured to transmit over a first wireless network; a second signal transmitter configured to transmit over a second wireless network; a storage device including the data application and a Voice over Internet Protocol handover proxy ; wherein the first and the second wireless networks are different types of wireless networks, wherein the first wireless network comprises a wireless local area network, a wireless wide area network, or Worldwide Interoperability for Microwave Access, wherein the second wireless network comprises wireless local area network, a wireless wide area network, or Worldwide Interoperability for Microwave Access, but is not the same as the first wireless network; and wherein the Voice over Internet Protocol handover proxy is configured to: relay the data application on the client device over an active connection to a proxy server and a second client device ; transmit media of the data application over the active connection; identify a seamless vertical handover event; maintain the active connection over a first wireless network to the proxy server and the second client device ; initiate Session Initiation Protocol re-invite messages to the proxy server while switching the active connection from the first wireless network to the second wireless network; and maintain the active connection and transmission of media of the data application over the second wireless network. relay the data application on the client device over an active connection to a proxy server and a second client device ; transmit media of the data application over the active connection; identify a seamless vertical handover event; maintain the active connection over a first wireless network to the proxy server and the second client device ; initiate Session Initiation Protocol re-invite messages to the proxy server while switching the active connection from the first wireless network to the second wireless network; and maintain the active connection and transmission of media of the data application over the second wireless network. <EOS>