If you have multiple numbers in your account, then you can 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 an outgoing call. Which number to choose, depends on what role (as an agent) you play and whom you are calling.
For example, in Exotel's case, our support number is 8088919888. If i am a part of support and if I am returning a missed call, then I could choose to select this as my number to make the call.
Also, if the customer does not pick up the call and later chooses to call the same number back, you might want the call to come to the same person or group. This person or someone from the same group can pick up the call and do the needful. In this case, once the customer calls back, the flow attached to that number you called from, will be followed. You can check the mapping of the flows and the ExoPhones here. Once chosen, Exotel remembers your last choice and always uses this number for all other outbound calls unless you explicitly change it again by clicking on the "Advanced -" link again.
Please note that if you have any Mobile Virtual Number(MVN) in your account, you will be able to choose the MVN to make the outgoing call, but the caller ID which will be displayed on the phone would be the landline number attached to the MVN.