T-1 (also known as DS-1) - This is the most common type of digital service provided by telecommunications carriers.  The T-1 is a transport service and often times must be coupled with an add-on service for use, especially in the case of voice service.  The T-1 can support 1.544Mbps (megabits per second) of data which can be used to deliver PRI service, Internet access, digital voice service or can connect two locations together for use as they choose.  The service is delivered over the same copper lines as POTS service.  However, because it is digital, there is a certain threshold of line quality that must be met before the T-1 is installed.  

T-1 lines are considered the gold standard of telecommunications services for most businesses.   In general, they are rated at 99.99% up-time.  The biggest disadvantage of T-1 service for use as a voice service is that if the T-1 is down, all of the lines on that T-1 are down.  This is not necessarily true of POTS service.  However, since T-1 service is a business class service, the response time of the carrier is typically faster than services like POTS or DSL.  Most companies that use T-1 service for voice services will have a few POTS lines as backup, giving them excellent redundancy at a reasonable cost.