QoS has absolutely nothing to do with wireless signal interference. QoS simply makes sure that certain types of traffic on the computer network get priority so that they are not interrupted by other traffic on the network. It does not have any effect on what happens outside the network, such as what the cordless phones and other sources of interference are doing...
As an example, QoS is like a room full of people talking at the same time and gives one (or a few) of them a microphone so that they can be heard over everyone else. It doesn't stop everyone else from talking, it just makes sure the important ones get heard regardless of what else is happening. It works great for traffic on the computer network, but has zero effect on anything outside the network. (I.E. No matter how much you turn up the volume on the PA system, it won't keep someone outside the building from knocking the building down.) It's not an exact analogy, but QoS and wireless interference really are completely different concepts.
Wireless interference is like having enough cars to fill two full lanes of traffic trying to drive in opposite directions at the same time in the same place on a one-lane road - it either won't work at all and nothing goes anywhere or there has to be a traffic monitor stopping one direction of traffic to let the other one through. Since there is no "traffic monitor" to make the phones stop talking while the wireless network is talking (and vice versa) the only way to fix this is to get each direction of traffic driving in its own traffic lane (channel).
Static on the phone while transferring files on the computer network means that the phone and the wireless router are on the same (or similar) frequencies within the 2.4GHz frequency spectrum and are interfering with each other. The only way to fix this problem is to eliminate (or at least reduce) the interference. As Spidey said, you need to force the router and the phones to use different wireless frequencies. There should be something in the phone's manual that says how to manually select the frequency/channel that it uses. If your router is set to channel 1, set the phones to channel 11. If the router is on channel 1, set the phones to channel 1. If your router is on channel 6, then 1 or 11 will work fine for the phones.
If there is no way to manually set the frequency on the phones, and the phones will not automatically switch to a different frequency than the one that is already being used by your router, then you'll just have to change the channel on the router to something else and hope that the phones won't automatically jump to that channel later on. If you can get the router and the phones on different channels, both should work properly as long as they stay on separate channels.
Having said that, there are some cordless phones known as spread spectrum phones that use all 11 of the 2.4GHz spectrum channels at the same time (or randomly jump between the channels) and if your phones fall into this category, then there really isn't anything you can do to keep them from interfering with your network and your choices are pretty much as follows:
1) Move all of the phone equipment (handsets and base stations) as far as possible away from the networking equipment (router and computers). You'll still have some of the interference, but it should be lessened slightly by the distance between conflicting devices.
2) Get new phones that are either 900MHz or 5GHz and won't interfere at all with your network, or are 2.4GHz and that will let you manually select what channel they use.
3) Switch to a 5GHz "A" wireless network for your computers.
4) Live with the fact that you'll have static on the phones and interference on the wireless network whenever both are used at the same time.
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