Tapi Calling ("Discon... method call on different web api")

Question

I've created restfull web api in C#.net for calling via tapi drivers.

•    Calling web api          (api/tapi/call). methoed for create call "CreateCall(....)"
•    Hangup call web api  (api/tapi/hangup).  method for hangup call "Disconnect(....)"

Case 1- if we call api/tapi/call with some post data then call connected to given contact no. then after we'll call web api api/tapi/hangup then generate error null reference.

 

Case 2-if we call api/tapi/call with some post data then call connected to given contact no. and "Disconnect(....)" method call in same web api then call disconnected easily.

 

Note -  How can we manage some state (object value) api/tapi/call data to api/tapi/hangup to.