Access control list can allow or deny ranges of IP addresses. There are several purposes for using the ACL.
providers - access conroll list
The main purpose of the Providers list is for your voip provider (carrier) IP addresses to the CIDR.
Be careful with what and how you use ACL.
Most common mistakes result in calls not working between extensions and other undesirable results.
Be sure to keep providers access control (formerly called domains) to default deny.
Do not put your public ip or phone IP addresses in the domains access control list.
Don’t supply both the domain and the cidr on the same node.
If adding a single IP address to the CIDR field make sure to add /32 on the end of the IP address.
Access Control Example
Goto Advanced > Access Controls. Click the edit icon for providers was renamed from domains. At the bottom under nodes click the plus icon. In the example below make sure to change the 188.8.131.52 to your VoIP providers IP address if adding to the Providers access control list.
Type choose allow
CIDR enter the 184.108.40.206/32
Domain (Leave Blank, used for advanced scenarios)
Description (Carrier Name)
Goto > Status > Sip Status and click reloadacl.
Under Status > log viewer you should notice the ip added. This can be seen also from command line fs_cli by using reloadacl
[NOTICE] switch_utils.c:545 Adding 220.127.116.11/32 (allow)  to list domains