A remote worker using the Avaya Communicator on the smart mobile phone obtains a private IP address delivered from the c

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A remote worker using the Avaya Communicator on the smart mobile phone obtains a private IP address delivered from the corporate network over WiFi when in range of the wireless hotspot. When the remote worker is out of range of the corporate WiFi it obtains a Public IP address via the 3G/4G mobile Service Provider.
When roaming from the public network to the corporate private network and vice versa, which strategy prevents the user from having to change the SIP Proxy
Server address in the smartphone?

A. Network Address Translation (NAT)
B. Avaya Session Border Controller for Enterprise (SBCE) public IP-address
C. Split Horizon DNS with FQDN
D. Virtual Private Network (VPN)