4f55f5cb_ISDN_Cause_Codes__Cause_Usage
[Cause#] 21 [Fixit!] N/A [Cause Summary] Call rejected. [Cause Usage] 
Indicates that the equipment sending this cause does not wish to accept this call, although it could have accepted the call because the equipment sending this cause is neither busy nor incompatible.
ISDN Cause Codes
Cause Usage