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: - (S1) receiving, at the cloud service server , an association request from a user account , the association request comprising a PBX extension telephone number; - (S2) sending, from the cloud service server , an association code to the user account ; - (S4) 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; - (S6) 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 ; - (S8) receiving, at the cloud service server through the IVR application , an association code from the PBX extension ; - (S9) comparing, at the cloud service server , the association code received through the IVR application with the association code sent to the user account ; and - (S10) 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 . - (S1) receiving, at the cloud service server , an association request from a user account , the association request comprising a PBX extension telephone number; - (S2) sending, from the cloud service server , an association code to the user account ; - (S4) 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; - (S6) 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 ; - (S8) receiving, at the cloud service server through the IVR application , an association code from the PBX extension ; - (S9) comparing, at the cloud service server , the association code received through the IVR application with the association code sent to the user account ; and - (S10) 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 . <EOS>