Just like you can make an outbound call to a number and connect your (User's) number to your customer, you can also dial a number and connect it into a Call App.
1) To do that login into your Exotel account
2) Click on the 'Call' button on the top left of the page
3) Enter the number you wish to dial
4) Click on 'Advanced +' and choose your required flow in the 'Connect To' dropdown.
5) Click 'Call'
The dialled number will receive a call and once s/he picks up, the call will be connected to the flow.