To replace Caller ID with the custom data do the following:
For example, if you set the value of Replace Caller ID to 'USDID' it will display 'USDID' on your phone display, for all calls coming through this DID.
Providing 'USDID %CALLERID%', will display 'USDID John 55510205' on your phone display, where 55510205 is caller's phone number and John is caller's name.
Note that the variable %CALLERID% contains the current call's Caller ID ("name" and <number>). %CALLERIDNAME% and %CALLERIDNUM% can also be used to get the current name and number independently.