This type of provider generally use the public Internet, where call quality might be spotty and DSL or cable lines can go out from time to time. This is fine for consumer use, but  our clients expect perfect calls and non-stop dial tone. That’s why we connect to our customers via a private connection.
Keep in mind that, Vonage, cable companies, and Skype are primarily selling to individuals, Mitel provides a virtual PBX service to business exclusively.