All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Apparently, there is no harm in you retaining this job because your work essentially consists in calling people and redirecting their phone calls, as described in the question. The work that you do and your contract with the firm have no relation with setting up on-hold music, and it is not the purpose of your job. Moreover, this on-hold music is one of the things that have, unfortunately, become quite widespread.
Hence, the salary you earn from this work is lawful because you receive it in return for carrying out permissible work which is making and transferring phone calls.
Allaah Knows best.