If no one answers or the line is busy, the dialing sequence must be repeated to effect call forwarding.
VOIP and cable telephone systems also allow call forwarding to be set up and directed via their web portals.
In Europe, most networks indicate that unconditional call forwarding is active with a special dial tone.
In North America, the North American Numbering Plan (NANP) generally uses the following vertical service codes to control call forwarding: The Sprint Nextel cellphone company uses these:[3] Most EU fixed-line carriers use the following codes based on CEPT and ETSI standards developed in the 1970s on both POTS and ISDN lines.
This is specified by inserting "*SC*XX" prior to the final "#" of the forwarding command, where "SC" is a service type code (11 for voice, 25 for data, 13 for fax), and "XX" is the number of seconds in increments of 5 seconds.
The main alternative is an answering machine or voicemail, but some callers do not wish to leave a recorded message, but want to have a two-way conversation.
Before the availability of call forwarding, commercial answering services needed to physically connect to every line for which they provided after-hours response; this required their offices to be located near the local central exchange and be fed by a huge multi-pair trunk in which a separate pair of wires existed for each client subscriber.
As a means to obtain an inbound number from another town or region for business use, remote call forwarding schemes tend to be far less expensive than foreign exchange lines but more costly than using voice over IP to obtain a local number in the chosen city.
Call forwarding can also assist travelers who do not have international cell phone plans and who wish to continue to receive their voicemails through VoIP easily while abroad.