In the Connect applet, in the "Dial Whom" section, there is an option to "Dial phone number (or phone number returned by this URL)". You can specify a URL in the text box. When the text box contains a URL, Exotel will make a HTTP GET request to that URL. The URL should return a number in the HTTP response. This is the number that Exotel will now try dialing and connecting the caller to.
For detailed list of parameters, refer to the updated documentation here.
NOTE: This option will be available only if your KYC docs have been submitted & approved.