I started out liking vbuzzer for voip. You can use their windows software or program a sip based VOIP adapter for use with their service.You pay as you go, ie: you can deposit, $5, $10, or $20 into your account and place calls till it is gone. It's only $2/mo for an incomming phone number (from a limit number of US and Candian area codes).
When I first started with vbuzzer that had exceptional rates for calling landlines and cellulars in Colombia. However, the rates have increase significantly, especially for calls to cellular.
I find they are no longer competitive for calls to Colombian cellulars, but still do better than others for landlines (though nothing like when I started). So, I have line 2 of my VOIP adapter programmed for Gizmo. I use vbuzzer for calling landlines in Colombia and Gizmo Project for calling cellulars.
The voice quality on Gizmo Project is much better when calling Michigan.
This is the advantage of having an open SIP adapter with two lines!