Really, why not use Skype? It is free, it is widely supported, it is convenient, it is already installed on lots of devices, users already know how to use it...
Do you know where a sensitive conversation is routed and stored?
Do you control the whole conversation?
Are you sure that all Skype messages, screen shares, file shares are encrypted throughout the channel?
Do you know if users' credentials and IP addresses are hidden from the outside world?