Bako, 36, will report to extended Spring Training in Clearwater, Fla.
Cairo, who the Phillies designated for assignment after Friday's victory against Washington, is expected to play for Triple-A Lehigh Valley, but it is uncertain when he will report.
"Paul has a tremendous amount of Major League experience and he gives us the opportunity to add greater catching depth to our system," Phillies assistant general manager Scott Proefrock said in a statement.
The Cubs released Bako on March 30, and he has not played since. Once Bako gets into game shape, he is expected to join one of the organization's Minor League affiliates.
Bako has hit .231 with 21 home runs and 186 runs batted in his 11-year career. In reserve roles with Kansas City, Baltimore and Cincinnati the past three seasons, Bako has not hit better than .217. He has thrown out 28 percent of potential base stealers (155-of-549).
Cairo, who had to accept the Minor League assignment as part of his contract, had to be designated for assignment after Philadelphia needed to use left-hander J.A. Happ in a 10-6 victory in 12 innings over Washington. Happ had been scheduled to pitch in the second game of a doubleheader Saturday.
But with Happ unavailable Saturday, the Phillies designated Cairo to recall right-hander Andrew Carpenter to pitch Game 2 on Saturday. The Phils then immediately optioned Carpenter for left-hander Sergio Escalona, who picked up the win Sunday.
Phils general manager Ruben Amaro Jr. said the club hasn't decided whether or not to keep Escalona or bring up another bat for its bench this week in Cincinnati.
Todd Zolecki is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.