Canīt help you with the specifics of the flight question (other than to say that Iīve found that it can be cheaper during the transatlantic low season to buy a separate ticket to Europe and then onwards to Africa rather than a straight US-Africa ticket).
As for cheap calling, Iīve found www.10-10phonerates.com to be really informative in terms of dealing with all the different dialarounds and figuring out which ones are cheaper than others. There are a couple dialarounds whose international rates are cheaper than the "special" rates for which MCI, Sprint, AT&T will charge you a monthly fee! (of course, none of these earns frequent flier miles, but with the exception of big sign-up bonuses, I donīt find these to be worth all the extra charges).
Yonatan