A live call, handed on
A call transfer takes a conversation already in progress and moves one of its ends to another line. A receptionist who says, “Let me put you through to accounts,” is performing one. The task sounds minor, but it needs equipment that can hold one connection while setting up another, and then join them. Humans on a switchboard did this with cords long before circuits could.
With an operator
On a manual switchboard, transferring a call was a matter of plugs. Suppose a caller asked the operator for a company, and the operator connected the call to the company’s main number, and the person who answered then said that the caller needed a different department. In some arrangements the operator could be asked to move the connection, and she would pull one plug and insert it in another jack. The cord pair stayed in place and only one end changed.
Such patching was routine in exchanges and hotel switchboards. A guest might be connected to the front desk, and the clerk would ask the operator to connect the call to a room. The skill involved remembering who was on which line, and sometimes announcing the call to the person who was being reached.
Private branch exchanges
Offices, hotels and factories built their own small exchanges, called private branch exchanges, or PBXs, to connect internal telephones and share a small number of outside lines among many users. In the manual PBX, an attendant at a board handled outside calls and internal connections, and transfer was an operator’s task.
Automatic PBXs allowed staff to dial each other’s extensions directly. The step from there to transferring a call without the attendant was a matter of signaling. A user on a call could briefly interrupt the connection, usually by pressing the switch hook or a button marked flash or transfer. The PBX would hear the interruption and place the outside caller on hold, then give the user a fresh dial tone. The user dialed the extension, and when the other person answered, hung up, which connected the outside caller to the extension.
The “hook flash” is a short break in the line, long enough to be noticed by the equipment but shorter than a hang-up. The same signal is the basis of other features that rely on a quick break, such as switching between calls.
Blind and attended transfers
Two styles became standard.
- Attended or consultative transfer. The person transferring speaks to the new recipient before completing the transfer, so the recipient knows who is calling and why. This is polite and avoids confusion.
- Unattended or blind transfer. The person dials the extension and hangs up at once, without waiting for an answer. It is quick, but if no one picks up, the caller may be left in limbo, though many systems return the call to the transferring extension if the destination does not answer.
Telephone etiquette developed around these options, including stating the caller’s name when transferring, giving the extension number in case of a disconnection, and not leaving callers on hold for too long.
Centrex and network features
Not every business bought its own exchange. Telephone companies offered a service called Centrex, introduced in the 1960s, in which the central exchange provided PBX-like features to a business, including internal dialing and transfer. A company could have short extension numbers and transfer calls without owning any switching equipment. This was an early example of switching functions being offered as a network service.
Digital and internet-based systems
Digital PBXs built in the 1970s and 1980s used processors and integrated circuits to control features, so transfer could be programmed, displayed on a screen and combined with other functions such as directories and busy lights. Telephones with a dedicated transfer key became common.
Internet-based telephony changed the method again. In systems based on common internet signaling standards, a transfer is a message instructing the other party’s equipment to connect to a new destination. This allows transfers across offices, between companies and across countries, with no special hardware. Hosted systems running on cloud computing services can do this for firms with no equipment of their own. Call centers and customer service lines use it constantly to route callers to the right specialist.
Mobile and consumer use
Mobile phones on cellular networks also let users move calls between devices, and on smartphones a transfer is just an on-screen button. A user may switch a call from a mobile phone to a computer or a car system, and conference features let users add a third person. These capabilities derive from the same principle: separate the endpoint of a call from the thing that is currently connected to it.
What the history shows
Call transfer is a good example of a task that began as a human skill and was gradually captured in equipment and then software. The principle, holding one connection while making another, has been unchanged since the operator’s cord, but the number of steps needed from the user has fallen to a button press or a tap on a screen.