Hi Shahzad,
1. Dont expose you SIP port on internet. If only for testing or local use, dont open it on internet. If you have to, use VPN.
2. Make sure all SIP/H.323 ALG on all Routers (they have one!) in your network is turn off. They can mask the intruder from outside and look like from local IP.
3. Change your default password! The one on vars.xml and every password on directory!
4. If you in control of your SIP client/ip phone, it good to change the default port 5060 and 5080 to some thing random (like 61351 etc). It make it harder for hacker to find your ports. Usually the SIP hacker is time and money oriented (not achivement-oriented), so most of the time it does not bother to find SIP port other than the default ports. They will quickly find another server IP to probe. They have organization behind them that can stream international phone call to the hacked servers. They usually test the softswitch first by sending alot of registration (some sip id is John etc). I think some show they can break the password using this because SIP authorization only using hash to check. So because hash has collision, they can calculate your password.
5. They usually try to break on weekdays (check your cdr) and use your hacked line on weekends! Please be carefull if you connected to PSTN line or operator! You can lose thousand of dollars in 1 day!
6. You can use SBC but you need very through on configuring. Unconfigured SBC same as no protection at all.
Lastly, the SIP world is VERY CRUEL. Honest mistake can destroy your life. Be EXTRA careful.
Donny