Calls that are repeatedly dialing in without anyone on the other end are known as ghost or phantom calls. Typically, they are repeated calls from an extension not on the system. Most extension are labeled ext. 101, 102, 103, etc. However, these calls might appear as 1001, 1002, and so on. Often, these calls cannot be answered.
They are usually generated by a third party who is trying to hack into the phone system, or they can be from a neglected auto-dialer. Cytracom's hosted service is completely secure, so there is no danger of them succeeding.
Changing the port that the phones use for SIP can solve this issue. If the calls still come through, then they can be blocked from the firewall. This is done by establishing one rule to block SIP traffic from all sources, and then add another rule with a higher priority to allow SIP from Cytracom servers.
In order to establish these rules, a business class router/firewall is a must!