I am looking for the chatting software that meets the following requirements:
(1) It should run on Windows, Mac and Smart phones.
(2) Only inter-office users would be allowed to use the app. Skype would be a good fit except users can include/invite anyone in the world who runs Skype.
(3) When the message is arrived, it will popup in a window or provide an alert with audible sound.
(4) Fyi, there is Outlook with Exchange already for the users. The reason for this chatting app is to let the users chat with other users on trivial stuff that don't need to be saved in Outlook/Exchange server.
I welcome commercial software as well.