It is illegal to play copyrighted music, or even radio, as hold or waiting music without the proper license.
In the United States and other countries, you must have a Music on Hold License to play copyrighted music for your callers. One of these licenses can be obtained from organizations such as the American Society of Composers, Authors and Publishers (ASCAP) or Broadcast Music, Inc. (BMI). These organizations represent hundreds of thousands of music creators and can issue the proper license to legally use music for your call center.
You can find an example of a Music on Hold license from ASCAP here.
We do provide some free audio files that are legal to use. Please check the article Can I change my hold music / ringtone options?