If you have multiple numbers in your account, then you could choose to make an outbound call from a number of your choice. Currently, everyone in your account has access to all the numbers in your account for the purpose of outgoing call. Which number to choose, depends on what role (as an agent) you play and whom you are calling.
For ex, In exotel's case. Our support number is 8088919888. If I am a part of support and, lets say, I am returning a missed call, then I could choose to select this as my number to make the call.
Lets say the callee does not pick up the call and later, calls the number back - I want the call to come to Support so that me or a colleague of mine in support can pick it up. Once chosen, Exotel remembers your last choice and always use this number for all other outbound calls unless you explicitly change it again by clicking on the "Advanced +" link again.