ANNISTON ARMY DEPOT, Alabama — Used in nearly every major US conflict since its introduction in 1980, America's mighty M1 Abrams serves as the principal battle tank of the Army and Marine Corps.

At close to 70 tons, the stalwart M1A1 Abrams is among the heaviest tanks in the world, but it makes up for its weight with exceptional firepower and unnerving maneuverability.

Today the Abrams has three variants — the M1, the M1A1, and the M1A2 — thanks to upgrades and modifications carried out at Anniston Army Depot, a maintenance and munitions-storage site nestled in the foothills of the Appalachian Mountains.

On a recent tour of the 3.5 million square feet of said installation, where war-weary tanks are resurrected, I was tipped off to inquire about a "grunt phone."

Combat vehicles inside one of Anniston Army Depot's repair facilities. Amanda Macias/Business Insider

Motioning for me to walk to the back of a turret-less Marine Corps M1A1, a logistics specialist at the installation named Larry Phillips pointed to what he called a "TIP," or tank infantry phone — aka grunt phone.

Secured to the rear of the hull underneath the taillight guard and above the track, the exterior phone housed in a box allows infantry troops to communicate with the tank crew.

Behold the grunt phone. Amanda Macias/Business Insider

The infantry phone, a frequent request from troops serving in Iraq, was incorporated in the 2006 rollout of the Tank Urban Survival Kit system, a series of Abrams modifications intended to improve coordination and survivability in urban environments.

Along with the grunt phone, TUSK added a remote weapon station machine gun operated from inside the vehicle, a loader's armor gun shield, reactive armor tiles fitted to side skirts, a remote thermal sight, and a power-distribution box.

Behold the grunt phone IRL. US Marine Corps

Because of upgrades and improvements such as TUSK, a new Abrams tank has not been built from scratch since 1993.