I hold no ill will towards marketing, but do become offended when someone feels I should lose my critical analysis skills and buy into marketing hype.
For example - revolutionary new technology allowing 1 digit calling? Easy connect to VoIP via cell phone? Yada yada...hype & emotion... &..well you get idea.
The way VoIP works, there are several critical aspects to insure anything close to toll quality calling. First is instrument used. Next is inside wiring. Assuming both of these are not a problem, access line becomes next major factor. Here there are 3 points. First is quality of access line - its ability to deliver error free bit streams. Next is bandwidth. As a shared application, bandwidth must be sufficient to serve needs of both VoIP and all other applications simultaneously. Third is logical distribution of that bandwidth. In effect, bandwidth must be split between VoIP and other traffic to insure VoIP has a consistent available rate of communications between premise and POP.
Getting all of above correct is not trivial, and those who attempt a pure plug and play approach stand a fair chance in being disappointed with their VoIP service. But let's assume this all goes well (at both ends for an end to end VoIP call). The next issue is travel over Internet from ingress POP to egress POP. Different carriers have different paths, which impacts latency. A VoIP session is probably most sensitive to latency of all common applications (although live video is probably most sensitive). The carrier incentive is to keep as much traffic as possible on their own facility. This is a business need, not a technical need, but it does impact quality of service users experience. Multiple carriers provides protection against failure, but it does not insure true shortest path routing. More important than multiple carriers is ability and willingness to purchase priority transport of packets based on IP header information or other protocol approaches, so that a VoIP call continues as a high priority session and with sufficient bandwidth across carrier's network.