The load balancing they describe in their article is only used for inbound requests, similar to a traditional HTTP LB setup. If I understand it correctly they don't implement the outgoing message delivery to the called user themselves, but instead use SMS or a Google Service to deliver push notifications.

So the problem of "how do I route request X to user Y" has to be solved by either Google or the Provider that delivers the short message. -- Actually, with their current setup they don't even need to know where a user is located geographically, since they simply choose a proxy server by asking Route53 for a list of close servers (I presume close to the calling user) and then use their connect-then-disconnect-hack to choose the "best" server from that list.

    > Instead we decided to write our own minimal signaling protocol 
    > and use push notifications (at first SMS, then eventually GCM when 
    > it was introduced) in order to initiate calls.

So I imagine you could just stick the address of the switch that was chosen by the caller into the message that gets delivered to the called device when a call is initiated.

Obviously this doesn't solve the hard part of "how not to drop a call when your switch goes down mid-call" though.