The settings on your router just need to match the ones on your SIP client/phone. If you're using Skype, I don't know how you do it in the client.
With SIP (as used by Plusnet) , the basic principle is that the phone marks packets with special header flags and the router spots these on the way in. If you set your router to give those marked packets high priority, it sends them ahead of all other traffic.
From memory, the Voyager has three queues corresponding to high/medium/low priority. Packets get sent out from lower priority queues only when the higher queues are empty. So set VOIP to high and nothing else to the same level and VOIP will get absolute priority over everything else.
AFAIK Plusnet ignore these TOS flags on their network and so it won't make any difference what flags you choose (as long as phone and router match) unless your VOIP provider is outside the PlusNet network.
