First of all...
Since Voice Over IP is basically using your internet connection to make phone calls, therefore your father's internet connect better be not fast but stable. Not sure if he is using cable or dsl internet but quality of the line must be fairly good and stable. Especially upload too.
If all works well, audio quality is better than landline, that is for sure. If not voice quality could be lower with few side effects like slight delay, audio breaking up & sudden disconnection.
Most importantly, power goes out... phone goes out unlike landline.