For VOIP (Voice Over Internet Protocol) to work well, all the routers involved in the connection must support COS (Class of Service).
Standard routers will degrade the throughput of "long" connections. This is older thinking that long connections, and we are talking long in computer time...so it might be measured in seconds, indicate a large data transfer and quick response is not an issue. Short transactions would be considered interactive type of traffic and would receive a greater portion of the available bandwidth.
With COS routers, voice connections are recognized and not degraded and thus receive the maximum available bandwidth for the duration of the connection. Other issues involve route switching and packet sequence which are also mitigated with COS routers.
COS capable routers are replacing all older standard type routers, by attrition in many cases, so your VOIP connection quality will vary depending on the hardware involved in the connection.
Worked for a company and we did VOIP over an MPLS (Multiprotocal Label Switching) WAN (Wide Area Network) network that had solid VOIP support as one of it's design parameters. We had branches in Europe and they worked well on our VOIP system.
However, even with all the right gear, there could be an occasional dropout or delay. We used standard phone connections for any customer calls, inbound or outbound....the thinking being we wanted 100% quality where the customer was involved rather than the 98% provided by our VOIP system.
BTW, for all of you who are grimacing at all the Acronyms in all this, the one I like the best is PCM-CIA. It is the name of the little card slot in the side of most laptops. I never could remember what the letters meant and had enough trouble remembering the letters themselves untill I found this mnemonic People Can't Memorize Computer Industry Acronyms.