Here in the UK I know that at least BT broadband modems set up a public Wifi, which is segregated from you own but uses you VDSL link for backhaul.
It looks like this is the same thing.
It looks like this is the same thing.
And besides, you can disable the BT 'sharing' capability. May not be so easy with an Echo in the house...
Ah yes, this is from the Echo so the traffic will go over your own Wifi.
BT also uses your bandwidth but it is the one over VDSL.