mimic a sip vicious call to phone behind a firewall

We are receiving calls that say from 12345@ourpublicip

There is nobody there.  Calls do not register in our hosted pbx that is on another public IP.  They are coming in right through the firewall. 

I have read Polycom's fix for requiring the exact registration server in the uri request, like this: 

voIpProt.SIP.strictUserValidation="1"
and 
<voIpProt.SIP.requestValidation voIpProt.SIP.requestValidation.1.method="source" voIpProt.SIP.requestValidation.1.request="INVITE" />

I want to us something like sipsak to mimic the calls so I can test the new configuration.

Does anyone know how to send a call to something like sip:1000@188.34.32.234 to mimic the spoofers that are getting in?

Thanks