VOIP needs added latency when done over a mobile connection. Digital mobile connections aren't the most stable or reliable. I can ping Google from my phone and get results anywhere between 30 and 300 ms. With latency being all over the place, you have to add a significant buffer to avoid audio cutouts.

And personally, I'd rather have a little latency added than have to talk over a shitty POTS line. I always had a hard time understanding people over it.