Look in the help in manager it is actually pretty good.
Modem Enabled Default = Off
The first analog trunk in a control unit can be set to modem operation (V32 with V42 error correction). This allows the trunk to answer incoming modem calls and be used for system maintenance. When on, the trunk can only be used for analog modem calls. The default system short code *9000* can be used to toggle this setting.
For the IP500 ATM4U-V2 Trunk Card Modem, it is not required to switch the card's modem port on/off. The trunk card's V32 modem function can be accessed simply by routing a modem call to the RAS service's extension number. The modem call does not have to use the first analog trunk, instead the port remains available for voice calls.
The truth is just an excuse for lack of imagination.