Since we have a errors code in PSTN call (IP -PSTN call) which we wanton our telephony server to pass those errors code directly to mobile apps. When subscriber's balance is lower than some predefined value during call setup, Warning header with SIP 180/183 response for low balance alert goes to calling Party (mobile client) and the call continues. If heart-beat with calling dialog fails or maximum time limit for the call is reached, Warning header with BYE message goes to calling party (mobile client) and the call is dropped. Other than this, following failure scenarios may hit during a call 1. Authentication failed 2. Account temporarily deactivated 3. Zero or inadequate balance while making call 4. Account already in use by configured number of users 5. Rate not defined 6. Called destination blacklisted 7. Any other application failure For all such failure scenarios, IN will send 408 SIP responses to client. This response will contain a Warning header specifying cause of failure and appropriate text message (configurable at IN). Format of Warning Header is as follows Warning: Cause_Code APP_Name "Error Text", e.g., Presently error codes are configured at network provider end which we need to pass from our telephony server to mobile apps