The Caller-id numbers are available on the following plans:

Enterprise SIVR

There is no restriction on the number of Caller-id that you can purchase on a specific plan. For example: If you are a Premium plan user then there is no restriction that you can purchase 4 or 5 caller-id number only. 

The caller-id numbers are not available to the Basic plan users. 

You can also refer to the below mentioned presentation which tells you about on which plans is the Caller-id number available.