Fixed Dialing Numbers on Windows Mobile 6.5

Fixed Dialing Numbers (FDN) in Windows Mobile 6.5 was a SIM-based security feature designed to restrict outgoing phone calls and text messages to an authorized, pre-approved list of contacts. Managed directly through the operating system's telephony settings using the SIM card's PIN2 code, the feature provided granular administrative control over cellular usage, making it particularly useful for corporate-issued devices, fleet management, and parental restrictions, all while keeping emergency calling accessible.

Core Architecture and Authentication

Unlike software-level call-blocking tools, FDN operated at the hardware and SIM firmware level. Windows Mobile 6.5 interacted with the SIM card via standard GSM AT commands and Telephony Application Programming Interfaces (TAPI).

To configure or enable FDN, the operating system required the user to enter the PIN2, a secondary security code provided by the mobile network operator:

  • Storage Location: All authorized numbers were stored directly on the SIM card's internal memory, not in the device's RAM or flash storage. This ensured the restrictions persisted even if the SIM was swapped into another phone or the Windows Mobile device was hard-reset.
  • Capacity Limits: The total number of permitted entries depended entirely on the physical SIM card's storage capacity, which typically ranged between 10 to 50 entries.

Configuration Process in Windows Mobile 6.5

Configuring FDN on a standard Windows Mobile 6.5 Professional or Standard device followed a direct path through the interface:

  1. Navigate to Start > Settings > Personal (or System) > Phone.
  2. Select the Security or Services tab (exact placement depended on whether the device used the standard Microsoft interface or a carrier/OEM customization).
  3. Select Fixed Dialing and tap Get Settings.
  4. Enter the PIN2 to unlock the configuration menu.
  5. Toggle the feature to Enable and add, edit, or delete numbers from the approved list.

Behavioral Rules and Wildcard Support

Once FDN was active, the Windows Mobile dialer altered its standard behavior:

  • Outgoing Calls: Users could only successfully dial numbers present on the FDN list. If an unauthorized number was dialed, the system immediately dropped the call attempt and displayed a dialog stating that calls were restricted.
  • Wildcards: Windows Mobile 6.5 supported prefix matching (wildcards). For instance, adding an area code or corporate prefix followed by blank spaces allowed users to dial any extension or local number matching that specific prefix.
  • SMS Messaging: Sending SMS messages was tied to the FDN list. If the recipient's number was not on the list, the outgoing message was blocked.
  • Incoming Calls: Incoming calls remained completely unrestricted. The device received calls from any number unless external carrier-level barring was active.
  • Emergency Override: By GSM standards, emergency numbers (such as 112, 911, and 999) completely bypassed FDN rules. Windows Mobile 6.5 allowed these calls to go through without requiring the numbers to be pre-programmed into the FDN directory.