Comment 13 for bug 1691556

I had the same issue with Device or resource busy and the same problem when Alexa says that is connected to pc and no sound - but it's because your pc becomes bluetooth speaker for Echo, in that state pacmd list-cards shows:
...
a2dp_source: Przechwytywanie o wysokiej dokładności (źródło A2DP) (priority 20, available: yes)
a2dp_sink: Odtwarzanie o wysokiej dokładności (odpływ A2DP) (priority 40, available: no)
off: Wyłączone (priority 0, available: yes)
...

Finally I got connected Echo as bluetooth speaker and it's working now after reboot too.

Steps to get Echo working as bluetooth speaker that works for me:

1. Echo doesn't know anything about bluetooth connection - so first forget your device in Alexa App, and other devices.
2. Reinstall your pulseaudio-module-bluetooth: for fedora: sudo dnf reinstall pulseaudio-module-bluetooth
3. Uncomment MultiProfile in /etc/bluetooth/main.conf and set it to multiple. (not sure if it is nesesery)
4. If you made modifications do:
sudo systemctl daemon-reload
sudo service bluetooth restart

5. Check if A2DPSink and source is registred with sudo service bluetooth status
6. If not do:
pacmd load-module module-bluetooth-discover
7. Reset adapter:
sudo hciconfig hci0 reset
8.Check if adapter works good: (my class is 0x1c0104)
hciconfig -a
9. Now unload pulseaudio modules:
pacmd unload-module module-switch-on-connect
pacmd unload-module module-switch-on-port-available

Now open terminal and run bluetoothctl, and in bluetoothctl type:
default-agent
remove ECHO_MAC (if you have saved it before)

Now tell to Echo "Alexa, bluetooth" and after "Searching" Echo should show in bluetoothctl so connect it:
connect ECHO_MAC
exit

After that bluetoothctl should show that connects to echo and pairs it, then pacmd list-cards will show a2dp_sink available yes. After reboot when you want to connect, don't say anything to echo, just run bluetoothctl and connect echo.

If after connect bluetoothctl doesn't show anything about pair and connect, you can try to manualy connect to a2dp_sink with:
dbus-send --print-reply --system --dest=org.bluez /org/bluez/hci0/dev_XX_XX_XX_XX_XX_XX org.bluez.Device1.ConnectProfile string:0000110b-0000-1000-8000-00805f9b34f
Replace XX with your echo MAC.

Fedora 29, BlueZ 5.50.