If the server is busy, the switch will attempt another connection. Without in any way diminishing the amount of intelligence that's being poured into these efforts, the big issue at the heart of all this integration is not technological. No, it's a cultural question. When I last spoke to one of the biggest telco equipment manufacturers, it was wryly admitted that, for all the amount of product available from this one vendor alone, not one large corporation had replaced its entire telecoms infrastructure with data network-originated kit. In many big corporations, the person who used to order the paper-clips has been promoted to the point where he now manages the office infrastructure, including the phones.
Phones are things that just work.
And telephone systems generally come from a single vendor, so there's no compatibility issues. The kit simply sits there, doing what it's supposed to, needing little or no attention. On this, the data networking side of the fence, there be dragons. We started life with our heads inside PCs and now we run the company's networks. Our kit seems to need vast quantities of management, and to be in a permanent state of turmoil.