For convenience and cost I think Google Voice for Prepaid can't be beat.
Here's a comparison for prepaid: Google Voice (GV) Pennytalk (PT) Skype (SK)
Land Lines GV=.06/minute PT=.05/min SK=.09/min
Cel phones GV=.11/minute PT=.19/min SK=.15/min
I mostly call cell phones, so GV wins there big time, and they also have auto refill available. They have an app to call from your smart phone, and on Android, can intercept all your international calls and reroute it through GV automatically. Also you can call from a pc.
Vonage plans for calling DR cell phones suck, and kinda bad too.
With a minimum plan like $9.99/month it will cost you .18/min
with a Vonage World plan of $25.99/month it will cost .8/min
Thanks, but no thanks!