score:6
Operators Lyca and Lebara are the best bet for your usage. Currently both of them charge around 4ct per minute to mobile and 1ct per minute to landline. I've seen couple of operators claiming to be lower than the above two, but they come with terms and conditions. At the end, Lyca/Lebara turns out to be cheaper.
Here are few tricks you can try to save few cents -
Upvote:1
Try Rebtel. Unlike other VOIP calling services, the way Rebtel works is that it connects over a normal phone call locally, then connects to the destination country over the Internet, and then terminates over a local phone network again. This way, you don't use data and you only pay for local minutes (plus Rebtel fees), which should be cheap enough on pay-as-you-go. Another such service called Jajah exists too which I've used, but Rebtel is much easier / has a better interface.