The second link posted is in specific to Webex Connect. The API specific to Webex Calling would be - https://developer.webex.com/docs/api/v1/call-controls/dial
With this, you can provide the destination you want to call, where that is a PSTN number, or an internal extension.
The destination to be dialed. The destination can be digits or a URI. Some examples for destination include: 1234, 2223334444, +12223334444, *73, tel:+12223334444, user@company.domain, and sip:user@company.domain.
Here is the full list of Webex Calling Call Control APIs available.
https://developer.webex.com/docs/api/v1/call-controls