This question (or variations of it) has been asked here a few times, and I have replied a few times. This is what I wrote this past week to someone who's iPhone kept connecting to Echo:
Too many BT signals on can cause issues, not just w/Echo. You need to turn off BT signals on your phones when not using them; you can't just leave them on constantly in the house and paired with say, the Echo..
I just told Alexa to turn off Bluetooth. She replied, "There aren't any devices connected. You can un-pair your Bluetooth on your mobile device." So, try that, and un-pair your BT from phone with Echo. You can re-pair when needed.
(The above also applies to Kindle tablets, iPads, etc. You also can likely just disconnect the Kindle BT from Echo.)