Method for associating a PBX extension with a user account of a cloud service application for controlling PBX extensions, the cloud service application for controlling PBX extensions being provided by a cloud infrastructure, the cloud infrastructure comprising at least a cloud service server, the PBX extension belonging to a PBX system, the PBX system further comprising a PBX central unit connecting PBX extensions to a Public Switched Telephone Network, the PBX central unit and the cloud service server being connected through a computer telephony integration - CTI - link over an Internet Protocol network, the cloud infrastructure further providing an interactive voice response - IVR - application available to the PBX extensions over the PSTN network, the method comprising: - receiving, at the cloud service server, an association request from a user account, the association request comprising a PBX extension telephone number; - sending, from the cloud service server, an association code to the user account; - sending, from the cloud service server to the PBX central unit, a command to monitor the PBX extension associated to the PBX extension telephone number; - sending, from the cloud service server to the PBX central unit, a command to connect the PBX extension associated to the PBX extension telephone number to the IVR application, the command including a public extension telephone number leading to the IVR application; - receiving, at the cloud service server through the IVR application, an association code from the PBX extension; - comparing, at the cloud service server, the association code received through the IVR application with the association code sent to the user account; and - if the association code received from the PBX extension is the same as the association code sent to the user account, associating, at the cloud service server, the PBX extension to the user account, thereby enabling the control of the PBX extension from the user account through the CTI link. - receiving, at the cloud service server, an association request from a user account, the association request comprising a PBX extension telephone number; - sending, from the cloud service server, an association code to the user account; - sending, from the cloud service server to the PBX central unit, a command to monitor the PBX extension associated to the PBX extension telephone number; - sending, from the cloud service server to the PBX central unit, a command to connect the PBX extension associated to the PBX extension telephone number to the IVR application, the command including a public extension telephone number leading to the IVR application; - receiving, at the cloud service server through the IVR application, an association code from the PBX extension; - comparing, at the cloud service server, the association code received through the IVR application with the association code sent to the user account; and - if the association code received from the PBX extension is the same as the association code sent to the user account, associating, at the cloud service server, the PBX extension to the user account, thereby enabling the control of the PBX extension from the user account through the CTI link.