The first point is a basic one in that I am not convinced whether an enterprise can avoid a server. I am assuming that the Peer ID will be enterprise specific. If this assumption is correct then when a person outside of the enterprise wants to communicate with one that is inside, the outside person has to contact a well known entity, which for all practical purposes is an IP PBX. If this system is used to interconnect to PSTN, it almost assumes most of the functions and cost structure of IP PBX. So I am not sure the real advantage of the serverless architecture.
The second point is whether an enterprise will accept the peer discovery procedure from a social and privacy point of view. In the Microsofts system, the peer discovery protocol involves querying intermediate nodes on the status of the target peer. This means one or more intermediate nodes will come to know that peer A is planning to communicate with peer B. One can easily visualize scenarios when this information is sensitive even within an enterprise.
Given these issues, it is useful to understand why a serverless architecture is preferable or in other words, whether centralized servers are inherently evil. Historically PBX vendors have locked customers into their system by having a proprietary interface between the telephone sets and the PBX, thereby increasing the cost of replacing the system with another vendor. This does not mean that centralized servers are inherently bad, especially in the IP domain. Web servers and email servers have been sufficiently commoditized both from capital and operational point of view. In summary, I do not see the downside of an enterprise wide system that is centered on servers, as long as open interfaces are maintained.