To setup QoS to an external VoIP device, such as an Engin VoIP ATA box, follow the instructions below.
Access the router web configuration page, then go to Quality of Service settings. Click on “Enable the QoS Control” and change the Direction to BOTH. Under Index 1, enter in a name for the QoS Index (e.g. VOIP), set a bandwidth percentage (which specifies how much bandwidth the index will get when the network is congested), then click Advanced.

Click On Insert Rule. The purpose is to setup a rule that specifies that any traffic type that goes to the VoIP Box will get priority.

Click ACT to enable the rule. Then click SrcEdit to edit the source ip address of the VoIP ATA box.

Change the Address Type to Single Address. Change the Start IP Address to the ip address of the VoIP ATA device.

The ip address of the VoIP ATA device is now shown in the Source Address field. Leave the other fields at the default value of “Any”.

The new rule is now Active.

The QoS for external VoIP device has now been setup.