The idea
Call forwarding does one thing: when someone dials your number, the call rings at another number you have chosen. The idea is easy to state and was simple for humans to perform long before it was automatic. Turning it into a feature of the network took changes in how exchanges worked.
Forwarding by hand
In the age of manual exchanges, a subscriber who was going out could ask the operator to send calls elsewhere. The operator would note it and, when a call came in for the subscriber, would connect it to the other line. Doctors, for instance, might leave word at the exchange about where they could be found. In small towns operators often knew where people were and passed calls on without being asked.
Answering services provided a more systematic form of the same thing. A message center could take calls, and relay them to the person by telephone once the person called in. These services were human, and so limited by staff time and cost.
Businesses used manual and, later, automatic private branch exchanges to transfer calls between extensions within the building. That internal switching is related to forwarding but is not the same: it involves a person who is already on the call.
Why it was hard for early machines
The early automatic exchanges, such as step-by-step systems, set up a call as the digits were dialed, directly connecting the caller through a chain of switches to the line of the number dialed. There was no central memory that could say, “calls for this number should instead go to that one.” Changing where a number rang meant changing the wiring or using a special arrangement.
Some companies supplied systems that used extra equipment for this. A customer could rent a service in which calls were routed to another number by a device that the telephone company installed, but such arrangements were custom, expensive and uncommon.
Electronic switching makes it routine
The change came with computer-controlled switching. Once an exchange had a processor and memory, built with early silicon chips, it could keep a record for each line: “forward calls to number X.” When a call arrived for that line, the processor consulted the record and routed the call accordingly. This was a natural use of the stored-program switches introduced from the mid-1960s.
Telephone companies bundled these as custom calling features, which also included call waiting and three-way calling. Introduced progressively in the 1960s and 1970s in the United States and later elsewhere, they were sold as monthly add-ons. A subscriber typically activated forwarding by dialing a code and then the destination number, and canceled it with another code. Exact codes and availability varied by country and system.
The variations
Forwarding has several forms.
- Unconditional forwarding. All calls go to the other number, and the original telephone does not ring.
- Busy forwarding. Calls go elsewhere only if the line is in use.
- No-answer forwarding. The telephone rings for a set time, and if no one answers, the call goes to the other number, or to a voicemail box.
- Selective forwarding. Only calls from certain numbers are redirected, a feature that depends on identifying the caller.
These variations let businesses build simple call-handling arrangements, such as sending calls to a mobile phone after hours or to a colleague when the line was busy.
Costs and side effects
A forwarded call uses network resources for both legs: the call from the caller to the forwarding number’s exchange, and then from there to the destination. In many systems, the subscriber who sets up forwarding pays for the second leg, particularly when it is long distance or to a mobile phone. Customers who forgot that forwarding was switched on sometimes discovered calls were going to an empty office, or found a bill larger than they expected.
Forwarding also raised privacy and security questions. If someone else could set it up on another’s line, calls could be intercepted. Providers added safeguards, such as requiring access through the subscriber’s own telephone or a personal identification number.
Mobile phones and the internet
Mobile networks included forwarding as a standard part of the service from the beginning. The digital standards for cellular networks defined forwarding conditions such as busy, unreachable and no reply, with codes that could be entered from any handset. This made forwarding useful for travelers and people with multiple phones. Smartphones put the settings in a menu, so most users no longer need to remember the codes.
Internet-based voice systems took the idea further. A single number can ring on a desk phone, a laptop and a mobile phone simultaneously or in sequence. Voice assistants and some services allow users to set rules by time of day or by caller, and the idea of a number that belongs to a person rather than to a place is now familiar.
What it changed
Call forwarding was an early example of the telephone number becoming separate from the telephone. Once the network could redirect a call, a number could stay the same while the destination moved. That separation underlies many later services, from toll-free numbers that route to different offices to personal numbers that follow someone across devices.