Obviously, the husband spoke to the EX since he confessed to the you. You can't determine if the ex decided on her own to call you or if she was prompted.
But, the message here is that even AFTER he came home and confessed to you his story, he was talking to the EX.
The reason for the confession is that he couldn't or didn't get all of his story to come together. He originally planned to just go to the football game as his cover, but couldn't get the pieces of that story to work. A key player in his cover either didn't show or didn't agree to keep the story. IMO, he never intended to go to the football game.
And, obviously, he contacts his ex either at work or through his cell phone and has been for a long time. If a couple has separate cell phone plans, they still should go over it together each month just as a matter of financial responsibility. Don't look at it as checking on each other, although it does help to review for that, but as getting the most out of your cell plan, making sure you don't have any extraneous charges, etc.