Cisco IOS is absolutely not based on BSD - it is a proprietary kernel, and such that it even has a “userland”, a proprietary userland.
IOS XE is based on Linux.
Most of the voice stuff is Linux.
Perhaps you are thinking of Juniper’s JunOS, which is based on FreeBSD?
It is used in Cisco's email and web security appliance, which is also their hosted offering. This appliance was previously known as IronPort, before being acquired by Cisco.
Cisco IOS is absolutely not based on BSD - it is a proprietary kernel, and such that it even has a “userland”, a proprietary userland.
IOS XE is based on Linux.
Most of the voice stuff is Linux.
Perhaps you are thinking of Juniper’s JunOS, which is based on FreeBSD?