Forwarded To Number

Description

The Forwarded To Number (FTN) is a critical data element within the Home Location Register (HLR) and the Visited Mobile Switching Center (VMSC) or Serving GPRS Support Node (SGSN) for GSM and UMTS networks. It is part of the call forwarding supplementary service data stored in the HLR for each subscriber. When a subscriber activates a call forwarding service (such as Call Forwarding Unconditional (CFU), Call Forwarding on Mobile subscriber Busy (CFB), Call Forwarding on No Reply (CFNRy), or Call Forwarding on Not Reachable (CFNRc)), the network stores the designated FTN provided by the subscriber. This number is the target for the forwarded call.

Operationally, when an incoming call arrives for a subscriber and a forwarding condition is active and met, the MSC or GMSC (Gateway MSC) interrogates the HLR for the subscriber's service data. The HLR returns the relevant FTN associated with the active forwarding condition. The MSC or GMSC then uses this FTN to route the call onward. The FTN can be a Mobile Station International Subscriber Directory Number (MSISDN), a landline number, or a service number. Its storage and management are defined in the subscriber's service profile.

The FTN's role extends beyond simple redirection; it is integral to service interaction and precedence logic. For instance, if multiple forwarding services are active, network logic determines which FTN to use based on the subscriber's status and the defined service precedence. The FTN is transmitted between network nodes using Mobile Application Part (MAP) protocols, specifically in operations like InsertSubscriberData and ProvideRoamingNumber. Its accurate provisioning and real-time retrieval are essential for seamless call completion and a consistent user experience across different network conditions and subscriber states.

Purpose & Motivation

The FTN was created to enable basic call forwarding capabilities, a fundamental telephony service expected by mobile users. It solves the problem of missed calls when a subscriber is unavailable, busy, or chooses to redirect calls to another number, such as a voicemail system or another phone. Before such services, a call would simply fail to complete if the called party did not answer, limiting the utility and reliability of mobile telephony.

Its introduction in GSM Release 99 formalized a standardized, network-centric method for storing and executing call diversion logic. This was a significant advancement over earlier, more primitive switch-based forwarding, as it allowed the forwarding logic and target number to be part of the subscriber's portable profile stored in the HLR. This enabled consistent service delivery even when the subscriber roamed to different networks or was served by different MSCs. The FTN parameter provided the necessary data element to make subscriber-controlled call management a reality, forming the basis for a suite of call completion and personal mobility services.

Key Features

• Stored as part of subscriber service data in the HLR
• Associated with specific call forwarding conditions (CFU, CFB, CFNRy, CFNRc)
• Can be any valid destination number (MSISDN, PSTN, service number)
• Transmitted via MAP protocols between HLR, MSC, and GMSC
• Subject to service interaction and precedence rules
• Essential for implementing voicemail and call diversion services

Evolution Across Releases

Defining Specifications