If you have Unified Messaging set up on the PBX, the Log Watch & Ban feature can send an email to the Admin Email address (found under PBX Setup=>General) when an IP address is banned. Below is an example of what such an email will look like.
The IP address highlighted in Red is the actual offending IP that was banned.
The IP address range highlighted in Yellow is the full range of IPs owned by the ISP.
The subnet of the ISP is highlighted in Orange.
From the email you can also discern the country of origin, and a lot of other information that can be useful. For example, you might want to create an ACL rule to specifically ban a foreign subnet, or contact the ISP to report the attack.