T1 telephone lines trace their development to the telephone companies who needed an efficient way to interconnect central offices for transporting dozens or hundreds of simultaneous phone calls. The resulting T-carrier standard created the T1 and T3 line services and paved the way for higher level optical carrier services capable of transporting thousands of calls. Decades later, this technology was made available directly to businesses as a way to connect their internal telephone systems, called PBX, to the public switched telephone network.
Any PBX system that you buy today either comes standard with or has an option for a T1 line card. T1 has become the standard for digital telephone trunks, the way to combine multiple standard analog phone lines into a larger trunk line for a cost savings. When used this way, T1 lines are configured into 24 channels. Each channel can carry one telephone conversation. With 24 channels available, up to 24 separate phone lines can be accommodated. The PBX (Private Branch eXchange) system manages those 24 lines to make them available to potentially hundreds of individual desk phones.
Of course, you don't need to use all 24 channels to have T1 telephone lines make economic sense. The more you use, the cheaper per line it gets. Usually the break even point is somewhere around replacing 6 to 10 conventional analog phone lines.
A very popular variation of the T1 phone line is T1 PRI or ISDN PRI service. The PRI stands for Primary Rate Interface. It is a standard that keeps 23 of the 24 T1 channels available to carry individual phone calls. The remaining channel is used for signaling, control and data to support the other 23. By doing this, T1 PRI offers very fast call switching and data services such as Caller ID and ANI.
T1 PRI has become the service of choice for PBX telephone systems and call centers. If 23 incoming/outgoing phone lines are not enough for a large corporation or call center, you can add more T1 lines. Many large phone systems easily accommodate 2 to 4 T1 lines and can be upgraded for more as needed. Need a hundred or more phone lines? DS3 telephone service gives you the equivalent of 28 T1 lines, which is well over 600 simultaneous telephone conversations.
T1 and DS3 are both channelized services that split the total bandwidth of the line into multiple independent channels on a time sharing basis. This technology is often referred to as TDM or Time Division Multiplexing. It has been the standard for digital telephony in the public phone network for over 50 years.
A newer service, designed to specifically to support VoIP phone systems, is called SIP Trunking. SIP refers to Session Initiation Protocol, the control and switching scheme used in VoIP. SIP Trunks are based on transporting packets instead of channels. They use IP networking technology instead of TDM. SIP trunks run directly from your location to your VoIP service provider, who provides the means to interconnect with the public telephone network. Because it's a managed service there can be considerable cost savings.
Interestingly, either T1 PRI or SIP Trunking can be used with both standard analog or digital PBX systems or the newer IP PBX phone systems. So the question becomes, "what is the best choice for a particular business?"
The right answer is whichever service offers the lowest costs for high voice quality and high reliability when connected to your phone system. You need to take into consideration the monthly recurring line costs, per minute long distance charges, and any capital expense for interface equipment. There may be other considerations, too, such as toll free numbers, local dial tone, and DID or direct inward dialing, among others.
How do you sort it out and come to a simple purchasing decision? The best way is to compare your T1 and SIP telephone options and get complementary consulting help for your particular circumstances. There's no cost for this service, and the savings that come from picking the right solution can be substantial.
