Looking for a source for royalty-free music for use in "Music On Hold" (when you place callers phoning into your business on hold - any ideas for low-cost / no-cost options?

Looking for a source for royalty-free music to utilize for music on hold for a medical billing practice. I honestly wasn’t aware how much some of this professional systems cost, so I was hoping that perhaps there were other sources of free music to use on a phone system when callers get placed on hold.