That's a pretty weak internet connection. You should consider attempting to do some tests online specific for voip prior to purchasing any equipment or signing up with voipo.com since QOS won't help speed up your internet.
This is a type of test you can perform: http://www.whichvoip.com/voip/speed_test/ppspeed.html
You will want to see what it says about the upstream and downstream jitter.

Honestly, if you're going to want VOIP, you should consider pausing netflix and downloads while on the phone.