Zoiper will ring when it receives a SIP INVITE from any source. This is desirable in some instances but not always. How do you make Zoiper ring or respond only to accounts created in Zoiper? That is to say; ignore SIP messages not meant for your active account.
I use Zoiper in a fire-walled scenario all the time but experienced an exception when SIP traffic was being sent to my computer DMZed and to my surprise Zoiper responds to all traffic.