4f55f5cb_ISDN_Cause_Codes__Cause_Usage
[Cause#] 18 [Fixit!] N/A [Cause Summary] No user responding. [Cause Usage] 
Used when a user does not respond to a call establishment message with either an alerting or connect indication within the prescribed period of time allocated (defined in Recommendation Q.931 by the expiry of either timer T303 or T310).
ISDN Cause Codes
Cause Usage