Texas prison officials have found polygamist sect leader Warren Jeffs guilty of "a major disciplinary infraction" following an investigation into whether he violated policy by -- among other things -- preaching a Christmas day sermon from prison, a state spokeswoman said Monday.

Jeffs' phone privileges have been suspended for 90 days, added Texas Department of Criminal Justice spokeswoman Michelle Lyons.

While refusing to elaborate on the content of the conversations, Lyons said that Jeffs was found guilty of making conference calls on several occasions. "It was obvious to us he was talking to a group of people," she said.

The leader of the Fundamentalist Church of Jesus Christ of Latter-day Saints, Jeffs is serving a life-plus-20-year term in Texas for sexual assault. He was convicted last August of the aggravated sexual assaults of a 12-year-old girl and a 15-year-old girl that Jeffs claimed were his "spiritual wives."

The state criminal justice department announced in late December that it had initiated an investigation into allegations that Jeffs used a prison phone to preach to his congregation on Christmas.

Records show that Jeffs made two phone calls on December 25, said Jason Clark, a Criminal Justice Department spokesman.

"It would be a violation of the rules if the person called were to place the call on speaker phone or record the conversation. The Office of Inspector General has asked us to suspend the accounts of certain individuals on his calling list while they continue to investigate," Clark said. He declined to identify those people.

Texas inmates are allowed to call as many as 10 people who have registered with the offender phone system vendor. Calls can be up to 15 minutes in length, and offenders are limited to 240 minutes of phone time per month. All calls are recorded and monitored except those between an inmate and his attorney.

Jeffs remains isolated in protective custody in the state's Powledge Unit prison facility near Palestine, Texas.

"He has no cellmate. No prison job. And the only time he leaves it (his cell) is for a shower and recreation," Clark said.

The 10,000-member church that Jeffs heads is a breakaway Mormon sect that openly practices polygamy in the twin border towns of Hildale, Utah, and Colorado City, Arizona, as well as on its Yearning for Zion ranch near Eldorado, Texas. The mainstream Mormon church renounced polygamy more than a century ago.

Many sect members have disavowed Jeffs in light of his criminal convictions, while others are defending him and calling his conviction on sexual assault charges an act of persecution.

A long-time follower of Jeffs recently said he had been ex-communicated after admitting to having sex with his wife -- a violation of an order that the sect leader apparently issued from behind bars. The church member spoke on condition of anonymity because of the sensitivity of the issue.