You can do "Echo Test" call. Just dial 4443 if I remember correctly. On this call everything you're gonna say will be echoed back to you by passing thru VoIP.ms servers. So you get a real good picture at the quality of the service.
Edit: You can consult VoIP.ms wiki page »wiki.voip.ms/article/Dialing_Cod ··· TMF_test
Hope this helps!