I currently use voip.ms for outbound international calls. I noticed that once in while after I dialed the number I wouldn't hear any ring sound only dead silence, but destination phone indeed rang.
I tried both value and premium route and had the same issue.
It's irritating because with dead silence I wasn't sure if the connection went thru and often I hung up before the other side picked up.
I am having the same problem right now. Awaiting for solutions.