The 'call rates' is a war we've fought and won. the interconnect rate is dropping every year, so call rates will be cheaper. I dont think it plays such a major role in a users/company decision making. If you see it, most promotions today by the cell providers offer per minute (CellCs 99c etc), but they offer such nice phones!! My wife signed up for one of them contracts just for the phone!
Frankly, most of the providers packages nowadays add up to the same. I ported from MTN to Vodacom, and my bill still averages out to the same, although some months my Vodacom bill is slightly higher, but I get lots more for it.
It's time we start moving forward in technology, by the providers offering us new calling technology, like video calling, or offer us free calling to some family members on the same network, even if it is a limited amount of minutes. Also they must enable QoS for VoIP like mobile Skype/Viber/Fring.
Another thing: If a VoIP providers main source of income is by selling airtime, but they sell it cheap, how will they make profit and be sustainable?! Short term goals and fly-by-night business strategy. Designing packages must have a good balance between profitability, flexibility, sustenance, and after sales service. Rather make little more profit on airtime, and use the extra profit to market the service and provide excellent after sales support. If you're not making money, you'll hire monkeys and pay them peanuts, because thats all you have!
The reason why I ported to Vodacom and pay a higher bill is because (I believe) they have a larger 3G/Internet network, as I travel extensively, locally and internationally. Since porting, I've have more productivity as coverage was less of a problem, and I've generally experienced better connection and after sales service with Vodacom.
Personally, I would pay slightly more to obtain better service, more uptime and more confidence in dealing with a company whom I know will be here tomorrow when I need them. After all, you get what you pay for, right?