Here's the problem.
Even if you have QOS on your router or modem, it only applies to that peice of hardware.
Your traffic won't be treated with the same qos parameters when it traverses other networks on it's way to the end point...and the endpoint may treat the traffic equally to all other types of traffic as well.
That being said, it could still be somewhat helpful to do on your own hardware. It's just not "true" QOS, unless it's done on both ends in a dedicated network...for example a point to point circuit, or an MPLS network where routers on all ends of it are configured with identical QOS mappings.
One of the first things I would ask you, in regards to the problem you are having,..... what codec are you using for phone calls ? Do you have any other codec options with your service provider ?
Second...how much internet traffic is typically going on, while those 6 concurrent calls are taking place ?
There is the possibility that you simply don't have enough outgoing bandwidth to support the amount of traffic, periodically. While you might be able to make adjustments to save call quality, you might inadvertantly slow down all other types of traffic, trading one problem for another.
