I have been working on the open source softswitch FreeSWITCH http://www.freeswitch.org/ for almost 6 years now.
During that time I have seen SIP continuously evolve to try to cover its own shortcomings which all stemmed from the simple concept of "If we base it on HTTP, we can use proxys and never have to worry about media" Of course this is not true and the amount of complexity that is put into each SIP device is much too great which is probably why Cisco prefers its own lighter "skinny" protocol. Sadly they own Sipura and Linksys and have SIP on their devices using countless hacks that make interop a nightmare. I am sure you can do many of these same attacks on any brand of phone. There are much better reasons out there to curse Cisco for being involved in VoIP. =D
 

We work on an open source softswitch called FreeSWITCH http://www.freeswitch.org/
Blocked ports and content filtering can mess up Voice over IP traffic running on your broadband line which can be used as a free alternative to the "Digital Phone" services many providers offer. Some entire countries already do this type of thing like China for instance. There are ways around it using secure packets so the payload cannot be sniffed and other workarounds but it would be a huge pain if we had to do that inside the US.

We have an Open Source project called FreeSWITCH http://www.freeswitch.org/ that allows you to do this sort of thing with any computer running Windows MAC or most UNIX. It can be paired with traditional phones with a small analog adapter or a hardware telephony card from Sangoma http://www.sangoma.com./ But you could just get a software phone for free as well and play around with it.
 

When you create closed source code you have a much higher chance of flaws because your code can not tested nearly as much as open source can. As the leader of an open source project, FreeSWITCH http://www.freeswitch.org/ , I am fortunate to have a very large crowd of beta testers who help ensure our releases are as stable as they can be. If you are selling the application and never letting anyone see the source you run a very high risk of missing something in Q/A and releasing buggy software. When people pay for it the will get angry so I am not surprised such a suggestion is being made but I find it unpractical to enforce since if it "works right" is hard to judge in some cases besides maybe medical equipment or other situation where human lives are at stake. Blue screens of death are hardly an excuse to sue anyone.

Our project (FreeSWITCH) uses the MPL for the main application and BSD for satellite libraries that we create that can be used by other projects etc.

Once you decide to have open source code, it's more logical to stick with the fact that at least the core code is FREE and come up with ways to develop a product on top of it if you want to have something to sell. Otherwise it sounds like an "open source tax" and businesses do not like uncertainty. If they choose to use a code base they need to know it will always be available.