Phone numbers are the addresses of businesses, the identification number as well as the communication gateway of a person, organization or a business. A telephone number is a string of decimal digits that uniquely identify an address. The number identifies the destination point that a call is routed to. It may be connected to devices and services like faxes, modems, subscribers and Internet networks.Most telephone networks are connected to The International Telecommunication Network (ITU) that has a standardized format of telephone numbers. The entire number should be 20 or less and must begin with a country prefix. This is usually followed by an area or city code. The format and allocation of local phone numbers depends on local governments.

Phone lines are critical to business and business applications are a prime priority for telephone technology. Initially, businesses used to have a single telephone number to a main switchboard, and a switchboard operator would connect the call within the business. If the called party did not respond, the call would be transferred back to the switchboard. But technology has changed all that. With voicemail and more technology, businesses now use direct inbound dialing (DID), so that outside callers can directly call a certain person in a business directly.

Phone numbers were traditionally immobile, but this has been transformed by the entry of mobile phones. In many countries, the practice of number portability allows customers to transfer a phone number from one local exchange carrier to another or add a fixed phone line to a mobile, Cellular phone numbers change according to the service area and sometimes according to the service provider you have chosen.

The traditional telephone system (POTS) and mobile phone networks share a common global standard called E.164 that allocates and identifies any specific phone line. However there is no global standard for VOIP networks. There are often different, incompatible schemes when calling between Voip schemes that use short codes that are provider specific.