Information Transmission
A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile transmission when they are using fax machines. The call may use land line, mobile phone, satellite phone or any combination thereof. Where a telephone call has more than one called party it is referred to as a conference call. When two or more users of the network are sharing the same physical line, it is called a party line or Rural phone line.
If the caller's wireline phone is directly connected to the calling party, when the caller takes their telephone off-hook, the calling party's phone will ring. This is called a hot line or ringdown. Otherwise, the calling party is usually given a tone to indicate they should begin dialing the desired number. In some (now very rare) cases, the calling party cannot dial calls directly, and is connected to an operator who places the call for them.
Calls may be placed through a public network (such as the Public Switched Telephone Network) provided by a commercial telephone company or a private network called a PBX. In most cases a private network is connected to the public network in order to allow PBX users to dial the outside world. Incoming calls to a private network arrive at the PBX in two ways: either directly to a users phone using a DDI number or indirectly via a receptionist who will answer the call first and then manually put the caller through to the desired user on the PBX.
Most telephone calls through the PSTN are set up using ISUP signalling messages or one of its variants between telephone exchanges to establish the end to end connection. Calls through PBX networks are set up using QSIG, DPNSS or variants.
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