Quality of routes
Routes manage the path of a customer's outbound calls, while DIDs bring inbound traffic to the customer. They both take care of the transit of a SIP audio call from caller to callee, and have many of the same challenges to their quality in common.
White, black, and grey
The technical barrier for providing termination services (routes to PSTN) and origination services (DIDs that get calls from PSTN) is so low that in countries and regions where VoIP is under monopoly, or where a cartel of big companies control the market imposing hefty prices, the business opportunity is so compelling that a plethora of independent operators, of widely differing reliability and regulation compliance (or which are outright illegal) discreetly populate ...