Multiple IP Addresses Mapping to Internal Computers

I will describe this as best I can, and please let me know
if I'm not explaining this clearly.

I work at a PBX company and as a side job we build very
basic beginner networks for customers. One of our larger
customers, with a full IT staff, set up a Cisco ASA(?) or a
PIX(?) device that accepts multiple Public IP addresses to
the device and routes them to different devices inside their
network.

Their services is a T1 flex product, which is for voice and
data. The voice services are handled by an ISDN-PRI card in
a Vertical PBX. The other part is the data portion of the
circuit.

For their data they have been given 5 IP addresses to use.
For example, lets say that the public addresses are
66.44.22.10 - 15 .

Now, the PBX has a VoIP networking card that is used to
connect with a main HUB in chicago for inter office
extension dialing. The PBX voip card has been assigned an
IP address 192.168.20.20. They also have a voicemail system
(seperate computer) which has another internal IP address of
192.168.20.21.

The IP Manager has created rules/code(?) for me so that I
can access 66.44.22.10 to telnet straight to the VoIP card,
and I can do VNC to the vmail computer on 66.44.22.11.
My questions are, how did he do this? Is this inside the
router or was it the firewall? Is there a basic ruleset
that he used or is this vendor specific?

I have an understanding of basic networking(geez, I've used
Wireshark extensively to map out bottlenecks in networks and
advise customers where VoIP packet loss is occurring,
bandwidth analysis, etc.), but there are obvious large holes
in it. I think another problem is that I have not worked on
sonicwalls, cisco's or managed routers. I have set up
various small offices using Linksys and Netgears FVS
product, but I have never gotten deeper than basic needs.

We do have an Adtran in the office, a 1224R, and I find it
awful. The GUI is a nightmare and the command line OS looks
to be like Cisco OS which I know little to nothing of.

In what way should I supplement my knowledge? Best place to
learn more deeply about routers and firewalls?

On looking at a lot of different posts, it seems that I have
to execute a line of code to map a public IP address to an
internal IP address. I've done port fwding on Linksys
routers, is this similar?

Thanks for any help in setting me in the right direction and
let me know if I've described the situation correctly.

Respectfully,

Eric