PSTN uses a digital backbone these days; it's just supersized carrier-grade digital backbone, not 'what hacky software can we get on the cheap'. Phone companies

PSTN uses a digital backbone these days; it's just supersized carrier-grade digital backbone, not 'what hacky software can we get on the cheap'. Phone companies prioritise their voice traffic over their own backbone and have network admins monitoring it closely, whereas your skype calls are just 'more traffic from the proles' and get to compete with everyone else.