Trying to understand non-SIP device configuration

I am a novice and I inherited a “working” Trixbox - Asterisk - FreePBX system
Asterisk 1.2.22 - FreePBX 2.23

A. I have figured out that to create normal voicemail extension in FreePBX- you create their “extension” by 1)Add an Extension, 2) Select Generic SIP device
and enter the SIP settings and then use Endpoint Manager to link the phone MacID to the new extension… so far so good…

B. There are also about 3 fax extensions which were configured using Cisco ATA-186 (used to get the analog ports with dialtone on a digital IP system)

These 3 “faxes” have extensions in FreePBX. As far as I can tell these “extensions” were created by 1) Add an Extension,
but 2) Select Other (Custom) Device. When you use this option it does not allow SIP configuration. In fact the extensions don’t show in Endpoint manager. It is “outside” of SIP and supposedly allows FreePBX to use custom settings in Asterisk.

The thing I can see which is different on the FreePBX extension configuration is they each read Dial: CUSTOM/5726 or CUSTOM/6574 whatever the extension #.
(The normal voicemail users have a Dial: SIP/5707 matching their extension.)

The only place these Fax Extensions show up in Asterisk are in the file

The only lines referencing a specific extension read

exten => 5726,1,Set(__FROM_DID=5726)
exten => 5726,n,Goto(from-did-direct,5726,1

and further down…

exten => 5726,1,Macro(exten-vm,novm,5726)
exten => 5726,n,Hangup
exten => 5726,hint,CUSTOM/5726

If someone could explain exactly what those 5 lines do
maybe I could figure out how this is supposed to work.

Thanks in Advance

Tom Gilbert - TomBexarMet