I agree that a VOIP quality test is the first thing to do. If you get a good result with nothing else using bandwidth, then your line is good for VOIP but the result may be misleading. You will need to rerun the test while using bandwidth. (Download a file or YouTube video). If the quality is then poor, then you know you cannot use bandwidth while on the phone.
It is my understanding that QOS does not restrict bandwidth if you are not using the phone. It just guarantees the phone will have whatever the allocated bandwidth is when the phone is in use. If you do use QOS with only 768Kbps, you will see all non-phone internet activity slow when on the phone.