[asterisk-users] SIP is not working sometimes. IAX is working fine. Why?

[Crazy Boy]

   Hi,
  
We are using "Asterisk" in our office and using "XLite" as softphone and your service for making calls to USA. 
 
 When I am using SIP, Sometimes It is working fine. But, sometime, when i am trying to make a call to USA, it is telling that "I am sorry. That is not a valid extension. Please try again. Error No. 2". But, after sometime, its working fine again without doing anything or any modifications. My intercom is also working fine always. What is this error? Please tell me the solution.
 
 When I am using IAX, It is working fine always. What is the problem with SIP?
  
  Looking forward to your response.
  
  Thanks&Regards,
  Chandra.
 			
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