Skype and Google Voice are all much the same as each other. They use a ring and spoke method to deliver the bandwidth (i.e. they are using your bandwidth even for other calls whilst you are on) and also use low bit-rate codec’s so they can fit as many calls into the available pipe as possible. When you rely on bandwidth you have no control over (like your own, even for other calls) you get drop-out, jitter and ‘pop’s’ and when you use low bit-rates you get a reduction in fidelity which causes lack of recognition of the caller and lack of high and low-band frequencies make it sound like you are in an echo chamber at one end and talking through a pillow at the other.
It doesn’t matter which of these ‘free’ services you use they will all have these characteristics. No one can sustain providing something for free. That is, no one is that philanthropic.