We use have used Click4prepaid.com for overseas calling for a long time — cheapest we have found for calling Mexico & Philippines (15.5 cents per minute for Philippines).
Call quality is a lot better than those calling cards they sell in the Asian grocery stores, plus with those calling cards, they take away minutes if you get a busy signal or no answer. Click4prepaid does not do that. Refill your account at their website with a credit card, and immediately after a call, refresh the website and shows correctly the amount left in your account, and minutes never seem to expire. (no I don’t work for them — we have just benn very satisfied with them for several years)
calling card ping