It would be naive to think they pay the same price internally within the OS supplier that they levy for licensing in a competitors telephony products. No company can survive with a critical component totally controlled by a competitor. They will be systematically bled of money, and them stomped. There is also no comeback in a closed source environment. Big name software vendors fill their licenses with clauses that both deny anyone else the right to deal with their software bugs, and deny the right of the purchaser to expect problems to be fixed.
Absolute power is exercised over bug fixing.
Maybe that is why you can't get a show stopper bug fixed. You may think this is a scare story but unfortunately it is not. Ask the companies who committed to Windows NT on the Alpha how they are feeling right now. In an Open Source environment the software comes with rights to modify. The source code is not actually the important bit.