Have you refresh that node? GenieACS by default only fetches minimal information. If you want to automatically refresh a particular parameter tree, create a provision script (or modify the default inform script with this:
let now = Date.now();
let twoDaysAgo = now - (48 * 60 * 60 * 1000);
declare('InternetGatewayDevice.Services.VoiceService', null, {value: twoDaysAgo});
This will refresh the VoiceService tree if the cached values are older than 48 hours. Adjust that as necessary. Keep in mind that after you have explicitly set the necessary values, there should be no need to continually refresh that parameter tree.
You can use a modified version of the provision script here Problems Creating Instances from provisions - #4 by akcoder and string it together with an extension script that retrieves params from your subscriber management system necessary to setup voice service