The inside man busted in the Diamond District heist on Veterans Day exchanged more than 25 phone calls and text messages with the gunman who pulled the stickup on the day of the robbery, the feds said Tuesday.

One of the calls from suspect Rondu “Reef” Frisby was made from inside Watch Standard Jewelry just six minutes before he buzzed in the suspected robber, Leon Fenner, according to a criminal complaint.

And the pair spoke on their cellphones again minutes after the heist, the complaint said.

Frisby was busted late Monday night and pleaded not guilty in Manhattan federal court Tuesday to a robbery charge. He was ordered held without bond.

The clueless crook was also caught on surveillance video outside the West 47th Street business with Fenner and a third man who acted as a lookout shortly before the bold daylight robbery, which the feds said netted the crooks more than 20 Rolex and other luxury watches, jewelry and cash worth at least $600,000.

The seven-page complaint said Frisby — a pal of owner Daniel Mikhaylov — entered the eighth-floor shop shortly before the robbery and made a call on his cellphone to Fenner at 2:14 p.m.

At 2:20, the nattily dressed Fenner appeared at the door of the business, which was locked.

Mikhaylov, Frisby and two employees could see Fenner on a video monitor and believed he was making a delivery, Mikhaylov told cops, and Frisby then buzzed him in.

Once inside, Fenner produced two large yellow envelopes and said he was there to serve Mikhaylov with papers.

Instead, he suddenly pulled a black, semi-automatic handgun and demanded cash and bling while the lookout remained outside in the hallway.

At one point, Mikhaylov’s father, Boris, entered the shop, and employees yelled for him to call 911.

But Fenner pistol-whipped him and ordered everyone to the floor, the complaint said.

The pair then fled but left the envelopes — with Fenner’s fingerprints all over them — behind in the shop, allowing cops to ID the accused gunman.

Surveillance video after the robbery shows the gunman and lookout walking quickly away from the robbery scene.

Frisby was also charged by the NYPD with criminal possession of a controlled substance and criminal possession of marijuana after cops found drugs in his Harlem pad.

Frisby, Mikhaylov told cops, was a friend and customer who had come to the shop that day under the pretense of helping a friend buy a gift for a gal pal.

Cops flooded West 47th Street after the mid-afternoon heist, which occurred as tourists packed Fifth Avenue down the street for the Veterans Day Parade.