Hello Deepika,
This is common for anyone running FS in the cloud with discoverable ports. Lots of people just run scripts that crawl the internet in search of SIP servers. After getting one they try bogus invites to try and see if they can get calls through - if your server is forwarding to PSTN, you could end up with a bill in thousands of dollars in minutest. In my case, I use a simple IP Table in Ubuntu (more like an access control list) to define allowed and non allowed IPs.
Can someone please elaborate on a measure that inolves config level security because blocking out masses is not goot Internet Citizenry.
Jude