Loop start is a a signaling technique in which a party gets a dial tone by closing (shorting) a DC current loop.
In a loop start signaling system, the electrical loop between your telephone and your central office (CO) is normally open. When you pick up the telephone handset, it causes a circuit to be closed — creating a loop.
Loop start signaling is normally used POTS (Plain Old Telephone System) lines and key-switch systems.
The most common alternative to loop start is ground start.
Advantages and Disadvantages
Unlike ground start, loop start does not require accurate ground references between the Central Office and each telephone set. This makes Loop Start a less expensive system to deploy and support over long distances.
On the other hand, loop start systems are vulnerable to an issue where someone can call you at the same time as you are attempting to place a telephone call. This is known as “glare.” Glare is not normally a problem for individual telephones, but can create problems for high-volume trunk lines.