SIP is ubiquitous and works pretty well. It's probably at the core of all these video conferencing apps. You can do audio/video calls and IM with it (using SIM

SIP is ubiquitous and works pretty well. It's probably at the core of all these video conferencing apps. You can do audio/video calls and IM with it (using SIMPLE). The problem is being able to call people beyond your SIP servers. Companies don't want to allow it, probably because they don't want to have their service become a commodity.