Calls happening randomly

Hey just a quick question.
Using Polycom phones and FreePBX randomly people will get calls from extensions where the users do not make the calls. The calls ring once and disconnect. Has anyone ever seen this and is there anyway to fix this?

Thanks for your help.

Those calls are sent directly to your SIP device’s IP address, from automated scanning tools, by hackers looking for PBX systems to break into. Disable IP to IP calling on your device or change the local SIP port.

Any idea on how to disable ip to ip calling for Polycom 335 phones?