He didn't even hear it ring as he sat in a meeting with some of his St. Louis business partners, going over some legal documents for his restaurant there.
''I had a missed call. My agent called me back and asked if the Patriots reached out to me,'' he said. ''It kind of went from there.''
Six weeks later, the three-time Pro Bowl selection - who last appeared in the playoffs as a St. Louis Rams rookie in 2004 - is in the unlikely position of being just a game away from a Super Bowl berth that he once accepted would never come.
''That was the largest factor to be honest with you,'' Jackson said. ''I haven't been in the postseason in quite some time. It had to be the right situation. It wasn't one where I couldn't move on with life or I couldn't move from the game. It had to be one that benefited both sides. And I think this is one of the few opportunities that allow that to happen.''
After appearing in 15 games with Atlanta in 2014, Jackson was released shortly after the season and was without an NFL job. At 30 years old, an age when most running backs are considered well past their prime, Jackson was content to enter post-football life.
But when a franchise with four Super Bowl rings calls, it's worth hearing the pitch.
''Of course, you didn't think 15 or 16 weeks into the season it would come...Nothing happens perfect,'' Jackson, 32, said in the locker room on Wednesday as he prepared to play the Denver Broncos in the AFC championship game.
''But this time it seemed to work out the best for both parties,'' he said.
Forget playing, Jackson barely watched football after his last NFL game. He debuted with the Patriots on Dec. 27 and said that before signing with New England, the only time he watched a game this season was on Thanksgiving.
He finds himself in a largely unfamiliar situation.
After 2004, Jackson went 10 consecutive seasons without a winning season or playoff appearance with St. Louis or Atlanta.
It didn't stop his productivity, though. He remains the Rams all-time leading rusher (10,138 yards), and he had more than 1,200 yards and 12 touchdowns in his brief stint in Atlanta.
''I think his career speaks for itself,'' coach Bill Belichick said after signing Jackson in December.
Jackson said the ''by committee'' approach with James White and Brandon Bolden that was described to him in his initial conversations with New England helped him visualize a situation he felt he could succeed in.
It also helped that he had previously played under Patriots offensive coordinator Josh McDaniels when McDaniels was in the same role for the Rams in 2011. Jackson ran for 1,145 yards that season - at the time, his seventh consecutive season of at least 1,000 yards.
''It just makes the transition a little smoother, once you're affiliated and exposed to the offense,'' Jackson said. ''Because it is quite complicated.''
In his two regular-season appearances for New England, he rushed 21 times for 50 yards and a touchdown. The Patriots only ran the ball 14 times in their win over Kansas City last week, but Jackson had six of those carries for 16 yards.
Though he never thought he'd be in this position two months ago, Jackson said the most important thing to him right now is the support he's getting at home from his family.
''I think it made it a lot easier as far as them allowing me to come back and get back into the game of football,'' he said. ''They understand this is just an opportunity I couldn't turn down.''
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Follow Kyle Hightower on Twitter at http://www.twitter.com/khightower