I've seen posts regarding Callcentric, VOIP.ms.

I posted a question on the Broadband VOIP tech board:

A few weeks ago I had issues with outbound calls to 2 specific numbers. One would consistently go to the recipient's voicemail, the other would drop after one ring. I contacted my VOIP provider (VOIPo) and the fixed the issue by modifying the routes.

Is this common among VOIP providers?

What can they actually modify and how much control do they have over the route a call takes?

... and got this answer:

As with all providers they do choose the routes that your calls take.
The routes are generally chosen by what is called least cost routing. That means that in practicality, the provider tries to choose the route that cost the least and hopefully works.
When that route doesn't work for some, they simply put them over another carrier that maybe costs more or maybe has better access to the terminating point.

Mostly has to do with cost to the provider but as in any business in the world, cost is a major factor..

With respect to VOIPo, I have been around since the beta testing days and they have evolved into a first class VOIP provider and at one of the lowest cost...service, in my opinion, is second to none, as in many cases the owner of the company gets personally involved in any issue that routine support cannot solve..

No, I don't work for them..