This is most likely a sign of someone cheating with a coworker. It’s normal for a coworker to call from time to time. Even more so if they are calling for something work related. However, if you find that your partner is constantly talking to a coworker, and those calls are happening late at night, then there’s something up. Remember that your time in the evenings is “family time”. If you find that someone of the opposite sex is suddenly taking up that time, then there’s something wrong.