One major complication in implementing NAT ALG for voice and video devices is that the NAT ALG cannot inspect the contents of encrypted signaling messages. Encryption prevents the ALG from accessing the data it needs to modify to support NAT, causing issues with address translation and routing.

Port 8443 may be unreachable to the Expressway-E from the internet if the Unified Communications zone is down or if the firewall is blocking the port. The Unified Communications zone being down would prevent necessary communication pathways from being established, leading to port inaccessibility. Similarly, if the firewall is blocking port 8443, any inbound or outbound traffic through this port would be obstructed, rendering the port unreachable.

The media encryption modes that can be configured on an Expressway zone include Force encrypted, Force unencrypted, Best Effort, and Auto. Force unencrypted specifies that all media must be unencrypted. If the target system/endpoint is configured to use encryption, the call might be dropped unless it can fall back to unencrypted media. This mode is one of the configurable options for media encryption on an Expressway zone.

A transform in the Expressway server is used to change an alias that matches certain criteria into another alias. This feature is essential for adapting and routing communication addresses to the correct format or destination, ensuring proper connectivity and communication flow within the network infrastructure.

SRV records used for SIP and H.323 communications need to follow specific naming conventions. For SIP over a secure connection, the appropriate SRV record typically follows the format _sips._tcp.example.com, ensuring encrypted communication over TCP. For H.323, which facilitates video conferencing and VoIP, the SRV record _h323ls._udp.example.com is used for UDP location signaling, crucial for discovering and managing endpoints. Hence, these two options are valid and align with standard conventions for their respective communication protocols.