A caller-id number is a dedicated number from which your customers will be receiving the call.

For ex: If I have purchased caller-id number 9999999999 and I initiate a call to my customer, then my customer will recieve a call from 9999999999.

You can select multiple caller-id if you are an advance/premium/enterprise/enterprise SIVR customer. You can choose to make an outbound call from a number of your choice.