You are asked to provide access for an external VoIP server to VoIP phones in your network using private addresses. However, due to security concerns, the VoIP server should only be able to initiate…

You are asked to provide access for an external VoIP server to VoIP phones in your network using private addresses. However, due to security concerns, the VoIP server should only be able to initiate connections to each phone once the phone has logged into the VoIP server. The VoIP server requires access to the phones using multiple ports. Which type of persistent NAT is required?