There are a couple of different calling options available from the Webex app depending upon which licenses are given to the end users. For this end user, they have the ability to "Call on Webex" which will allow them to call anybody that is also using the Webex app. "Register to Unified Communication Manager (UCM)", this license allows the user to end user to use the CSF device in CUCM (which is also used for Jabber) to make outbound PSTN calls. And then there is "Webex Calling" where the end user's phone is registered to the Cisco Webex cloud through Control Hub and uses one the PSTN options available through Control Hub.
The web “app” is from what I know not a downloadable thing as such. I’ve never looked at what functionality that is available in the web option for Webex, but I would doubt that using it for calls would work in a reliable way as it’s a thin client with limited functionality. Your better of to use one of the installable options.
