1. A computerized system for providing unified communication services for communication sessions between two or more client devices, the system comprising: a Domain Name System (DNS) server coupled to a plurality of globally-distributed computing device clusters which are coupled to each other over a communication network, the DNS server configured to: receive, from a client device, a transaction request associated with a communication session, the transaction request including a transaction state of the communication session and a call state presence event corresponding to the transaction request and independent of the transaction state; and determine one of the plurality of globally-distributed computing device clusters that is capable of handling the transaction request according to a current load and a capability set of the cluster, and transmitting the transaction request to the determined cluster; wherein each cluster comprises: a Session Initiation Protocol (SIP) server configured to process the transaction request and including a SIP proxy module and a SIP registrar module for managing the transaction state associated with the communication session, the transaction state corresponding to a sequence of SIP messages exchanged between the client device and other SIP network elements, an event queue configured to extract the call state presence event from the transaction request for storage in the event queue, the event queue further storing messaging presence events received from an Extensible Messaging and Presence Protocol (XMPP) server of the determined cluster, the messaging presence events corresponding to availability status of a client device for non-call messaging, and a session state handler configured to retrieve the most current presence event from the event queue and notify either the SIP server or the XMPP server of the retrieved presence event; wherein the SIP server, XMPP server, and session state handler each stores and retrieves data associated with the communication session in a database shard at the determined cluster; wherein the database shards of each cluster collectively comprise a single logical database; and wherein the SIP server manages the transaction state separately from the session state handler retrieving the most current presence event and notifying either the SIP server or the XMPP server of the retrieved presence event.