Plain old telephone service (POTS)/ Business Line  is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a retronym, and is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN, mobile phones and VoIP because of its wide popularity as a reliable service solution. POTS has been available almost since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged to the normal user despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

The system was originally known as the Post Office Telephone Service or Post Office Telephone System in many countries. The term was dropped as telephone services were removed from the control of national post offices.

POTS services include:

The Integrated Services Digital Network (ISDN) : ( PRI & BRI) The Primary Rate Interface (PRI) is a standardized telecommunications service level within the Integrated Services Digital Network (ISDN) specification for carrying multiple DS0 voice and data transmissions between a network and a user. Primary Rate Interface (PRI) PRI is the standard for providing telecommunication services to offices. It is based on the T-carrier (T1) line in the US, and the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32. T1 and E1 systems 

PRI provides a varying number of channels depending on the standards in the country of implementation. In North America and Japan it consists of 23xB (Bearer channels) and 1xD (Data/control channel/) (23 64-kbit/s digital channels + 1 64-kbit/s signaling/control channel) on a T1 (1.544 Mbit/s). In Europe and Australia it is 30xB + 1xD on an E1 2.048 Mbps. One timeslot on the E1 is used for synchronization purposes and is not considered to be a B or D channel.


The Integrated Services Digital Network (ISDN) prescribes two levels of service, the Basic Rate Interface (BRI), intended for the homes and small enterprises, and the Primary Rate Interface (PRI), for larger applications. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64-kbit/s B-channels and one 16-kbit/s D-channel.

Larger connections are possible by using PRI pairing. A dual PRI could have 24+23= 47 B-channels and 1 D-channel but more commonly has 46 B-channels and 2 D-channels thus providing a backup signaling channel. The concept applies to E1s as well and both can include more than 2 PRIs. Normally, no more than 2 D-channels are provisioned as additional PRIs are added to the group.

 The disadvantage of PRI is the cost associated with the PBX at the customer premise. The Centrex solution resolves this issue by providing the PBX function from Carrier’s Central Office. Centrex is a PBX-like service providing switching at the central office instead of at the customer’s premises. Typically, the telephone company owns and manages all the communications equipment and software necessary to implement the Centrex service and then sells various services to the customer.

No switching equipment resides on the customer premise as the service is supplied and managed directly from the phone company’s exchange site, with lines being delivered to the premises either as individual lines over traditional copper pairs or by multiplexing a number of lines over a single fiber optic or copper link. In effect, Centrex provides an emulation of a hardware PBX, by using special software programming at the central office, which can be customized to meet a particular customer’s needs. As with a PBX, stations inside the group can call each other with 3, 4 or 5 digits, depending on how large the group, instead of an entire telephone number.

Centrex obviates separate exchange lines delivered to a site for use with a 1A2 Key System or similar, or PBX. Instead, telephone extensions, called Centrex lines, are delivered directly from the local exchange to the user. Newer IP PBX systems also allow phones at any location with a WAN or Internet connection to act as a local extension. Facilities such as Direct Inward Dialing (DID), where individual extensions are offered a direct and unique telephone number for incoming calls, are standard features in a Centrex environment. Stations may also be part of a hunt group, allowing for automatic distribution of incoming calls to two or more extensions.

Choosing the right solution

Selecting the right voice solution for your organization is very important for the successful daily operation of the business. We can help you in selecting the right local or long distance voice solutions at the right pricing

Depending on your requirement and your organization structure and roles, we can assist you to select the right way to implement the voice solution best suitable for your organization