Jagger was calling to discuss Gaga's collaboration with the Rolling Stones during their gig in New Jersey this weekend.
Gaga, who was in the bath at that time, came out and took the phone with her inside the bathroom.
"Gaga didn't want to miss Mick's call so she took it in the bath. They spoke at length about what's going to happen. She also asked Mick if she could wear any of his old stage gear for their performance," a source said.