I found year old coverage at Recode and PC Magazine:
I paired my device. That was simple, altho I had to try a couple times, and ultimately turned off bluetooth on my phone and turned it back on again then moved my phone even closer to Echo in order to convince it to show up as a pairable device. Once paired, I started music on the phone side. It was kinda quiet, so I turned up the volume a little, then went. Hmmm. "Alexa, please turn the volume up." That helped, and a few more, "Alexa volume up"s later, and I could hear it in the kitchen. The Echo is a perfectly adequate bluetooth speaker (that is, it's no AirPlay enabled Pioneer, but the Pioneer won't tell knock knock jokes, either).
My sister called me while I was playing music. Alexa rang. My phone rang. Much ringing. I answered on the phone, and it did not play over the Echo (I wonder if you can answer it through the speaker? Hmmmm. ETA: Apparently not. https://www.amazon.com/gp/help/customer/display.html?nodeId=201549660 "Note: Phone calls, text messages, and other notifications from your mobile device can't be received or read by Amazon Echo, and audio from Amazon Echo can't be sent to Bluetooth speakers or headphones."). When I ended the phone call, the music resumed.
Nice! This is kinda fun. I still wanna get some controllable lights, but this is pretty cool.