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 an HTTP GET request to that URL, and the URL should return a number in the HTTP response. This is the number that Exotel will try dialling, and connecting the caller to.
For a 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.
Feel free to write to firstname.lastname@example.org for any questions.