We have seen a increase in SIP hacking attempts across NZ at the moment, it comes and goes in cycles and you would have possibly seen a few News Items about it as well ,
Whilst we take care of the certified devices on VFX, security on Open VFX and Asterisk devices is the users' responsibilty so here is a nice little webinar sent to me by my support team to share with you.
Please Asterisk users take your security seriously as getting burned can be costly for you and us and niether of us want that

If in doubt seek advice
http://www.asterisk.org/security/webinar