Your Exophone is the number that your customers would see on their phone's called ID when you make a call to them. 

But sometimes when you make an out-bound call, there could be some connectivity problems from your Exophone to the customer's number. In such cases, the Exotel system automatically uses a redundant/backup Exophone to connect the call.  Only in such a  scenario, your customers would see a number that is different than your Exophone. 

I will explain this further with an example. Let's say you purchase the number 080-XXXXYYYY from Exotel for your business. You are also provided with redundant numbers such as 080-XXXXZZZZ, 080-YYYYXXXX. These redundant numbers are usually hidden from the system and only used when there are problems with the main Exophone.

When you make outbound calls your customers would receive a call from 080-XXXXYYYY (Your primary Exophone). This is the number they will see on their phone's Called ID. 

When 080-XXXXYYYY (Your primary Exophone) has problems connecting with the customer's number, then the call would go from 080-XXXXZZZZ (Your redundant Exophone). 

You may ask: What if the call gets missed and the customer calls back to 080-XXXXZZZZ (the Redundant Exophone) ? Then there is no problem because this redundant number acts like a clone of the 080-XXXXYYYY (Your primary Exophone) and all the options would work similarly!