Depending on which ESI phone you have, use the appr opriate diagram (below) to connect it. Each diagram represents the panel on the phone’s underside. Note: The “Top of phone” and “Bottom of phone” references in these diagrams show the correct vertical orientation of the phone — i.e., the part with the display is the top.
I haven't used this device for an extended period, but since there are no mechanical parts, I expect that it will last. We have been using a professional on hold service for many years at about $60/month. The box that the professional company gave us sounded great when customers would call from a Lan Line. But, when calling from cell phone the voice messages were lower volume and the music would sound horrible. Music keeps cutting in and out and sounds very staticky. So we decided to save some $ and cancel our subscription and buy this unit. We are using the default music and voice messages that came preloaded instead of doing our own music and messages.

So modern technology, even with the most advanced VoIP business phone systems, has not improved the plight of the on hold song. If you are interested in learning more, check out Tom Scott's explanation at the Milton Keynes Museum, in their Connected Earth gallery.
Someday somebody’s gonna put you on hold and make you cry… Cheer up, you’ll only have to hold on for one more day.