George Zimmerman trial:"It's Georgie" screaming on 911 call, wife of best friend testifies

George Zimmerman arrives for the 20th day of his trial in Seminole circuit court July 8, 2013 in Sanford, Florida. Photo by Joe Burbank-Pool/Getty Images

(CBS) -- The wife of a man who describes himself as George Zimmerman's best friend testified Monday morning that she believes George Zimmerman was screaming in the background of a crucial 911 call.

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"Yes, definitely. It's Georgie," said Sondra Osterman, the wife of Zimmerman's friend Mark Osterman, who also took the stand Monday as the defense continued their case.

The 911 call, placed by a neighbor the evening of the fatal confrontation between Zimmerman and slain Florida teen Trayvon Martin, was played several times in court Monday morning. The tape is a crucial piece of evidence because it could provide clues as to who was the aggressor in the fatal Feb. 26, 2012 confrontation. Both Zimmerman's mother and Martin's mother testified in court last week that they believe the voice belongs to their son.

Zimmerman, a former neighborhood watch captain charged with second degree murder, claims he shot the teen in self-defense.

Sondra Osterman choked back tears as she testified Monday. "I just hear it. I hear him screaming," Osterman said.

Osterman said she met Zimmerman in 2006 when they worked at the same mortgage company. 

VIDEO: Zimmerman trial: Prosecutor opens with profanity

On cross-examination, prosecutor Bernie de la Rionda asked Osterman whether she had a "stake" in the case because she and her husband wrote a book about Zimmerman. Osterman said she has "no idea" how many books have been sold and said the proceeds were being deposited in savings account to be donated to Zimmerman.

Also played in court Monday was a non-emergency call Zimmerman placed to dispatchers the night of the fatal struggle, during which he used the terms "these ---holes" and "f---ing punks."

Defense attorney Mark O'Mara asked whether she believed the terms demonstrated ill will or spite, elements the state must prove in order for a jury to convict Zimmerman of second degree murder.

"No," Osterman replied.

Sondra Osterman's husband Mark also said he believed the screaming voice belonged to Zimmerman as he took the stand the stand Monday.

Mark Osterman, an air marshal, said he talked with Zimmerman about what kind of gun to purchase and visited the firing range with Zimmerman. At the firing range, "firearm safety was always at the top of our list of discussion" and Zimmerman was "very safe all the time" with his gun, Osterman said.

He described Zimmerman's training with firearms in court, and said he advised Zimmerman to keep the gun loaded.

Osterman said he and Zimmerman discussed purchasing a gun without external an external safety in case he might find himself in a "tussle" and needed to defend himself quickly.

"Ironically enough, we had this discussion," Osterman said.

Osterman said he and Zimmerman practiced shooting with their "non-dominant" hands. Zimmerman, he said, shoots right-handed, but writes with his left.

In a self-defense situation, "whatever hand can get to the firearm, that's the one you can use," Osterman said.

Leanne Benjamin, a friend of Zimmerman, and former co-worker Geri Russo also testified Monday they believed the voice on the call was George Zimmerman's.