I think the answer is extremely simple to "Why hasn't Apple bought Voip-Pal". Voip-Pal has no income. It is banking on the strength of it's patent portfolio to stay in the game, and cause top tier lawyers to bet on the come, rather than get paid on the dotted line. Apple is betting that it can, with superior staying power, eventually outlast tiny little David with a sling-shot. They are banking on the fact that one little shenanagin on the part of a BOD that the team of lawyers didn't know about could upset the fine balance of agreement and the top notch legal team could walk away.
That's a pretty decent bet....Until, of course, this lawsuit gets going in earnest, and Apple is forced to display the potential damages as a potential liability on their balance sheet - I believe we'll see some new approaches once that happens. IMHO