A method for assisting in establishing an online communication session between client devices, comprising: receiving, by an invitation service, from an initiating client device, an invite request message to an intended recipient for an online communication session, the invite request message including connection data of the initiating client device and an online communication session endpoint identifier for the intended recipient, the initiating client device and a client device of the intended recipient having been previously registered in an online communications network by a registration server, the registration of each client device (710, 720) comprising storing in a registration data store (150, 655) an association between a communication session endpoint identifier and a unique push token (715, 725) that uniquely identifies the client device (710, 720); accessing, by the invitation service, the registration data store to determine the unique push token associated with the online communication session endpoint identifier of the intended recipient, wherein the push token uniquely identifies the client device of the intended recipient that has been previously registered in the registration data store for online communications thereby establishing that any notification message received from the intended recipient client device is based on a prior registration; transmitting, by a push notification service, an online communication session invite notification message that includes the connection data, endpoint session identifier, and push token of the initiating client device, to the intended recipient client device that corresponds with the push token of the intended recipient client device; receiving, by the invitation service, an invite accepted notification message from the intended recipient client device that includes connection data of that accepting client device and the push token of the initiating client device; and transmitting, by the invitation service a push request to the push notification service requesting transmission of the invite accepted notification message, received from the intended recipient client device, to the initiating client device, the invite accepted notification message including the connection data, session endpoint identifier, and push token of the intended recipient's accepting client device to enable the initiating client device and the accepting intended recipient client device to establish a direct peer-to-peer online communication session. receiving, by an invitation service, from an initiating client device, an invite request message to an intended recipient for an online communication session, the invite request message including connection data of the initiating client device and an online communication session endpoint identifier for the intended recipient, the initiating client device and a client device of the intended recipient having been previously registered in an online communications network by a registration server, the registration of each client device (710, 720) comprising storing in a registration data store (150, 655) an association between a communication session endpoint identifier and a unique push token (715, 725) that uniquely identifies the client device (710, 720); accessing, by the invitation service, the registration data store to determine the unique push token associated with the online communication session endpoint identifier of the intended recipient, wherein the push token uniquely identifies the client device of the intended recipient that has been previously registered in the registration data store for online communications thereby establishing that any notification message received from the intended recipient client device is based on a prior registration; transmitting, by a push notification service, an online communication session invite notification message that includes the connection data, endpoint session identifier, and push token of the initiating client device, to the intended recipient client device that corresponds with the push token of the intended recipient client device; receiving, by the invitation service, an invite accepted notification message from the intended recipient client device that includes connection data of that accepting client device and the push token of the initiating client device; and transmitting, by the invitation service a push request to the push notification service requesting transmission of the invite accepted notification message, received from the intended recipient client device, to the initiating client device, the invite accepted notification message including the connection data, session endpoint identifier, and push token of the intended recipient's accepting client device to enable the initiating client device and the accepting intended recipient client device to establish a direct peer-to-peer online communication session.