understanding IP restrictions

Just so I understand IP restrictions clearly, I have a client with 3 remote sites,  In the firewall I have setup a rule to prevent the system from blocking those IP addresses.  Occasionally, we get hit with a bogus ip address that connects to one of the phones and causes havoc.  

All three locations have a static IP address.  Is it safe to assume that if I add the static IP address to each phone at each location, then those phones would only register and allow calls from the restricted IP address?

Here is what is happening that I want to prevent:

Ext 101 is located at static IP address of

I looked at the monitor to see that  IP: was also attached to the phone which rendered the phone useless.  If I restrict the IP address to it should then only allow connection and registration from that IP address.  Correct?


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