When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I believe that relay allows the telephone speaker sound to come through the audio speakers during a telephone call. Or something similar depending on how it is set up to work on your vehicle. Sometimes it is meant to mute the stereo when a telephone call comes in or is made. pretty basic stuff though.
Unplug the thing and see if all your speakers still work. If they do then you don't need it. I would also totally remove the phone if you're not using it.