I did something similar with my Dad. He's in Yorkshire and I'm in Kent, has broadband and an old tablet. We downloaded the app onto his tablet and installed his Echo, registered it to him, so now he doesn't need to do anything except say "Echo call Martin" and it calls me. I can do the same my end saying "..... call Dad". We've had no issues so far and it's been about 3 months now. The voice connection is reasonable with the occasional drop out but I suppose that depends on BB speed etc.
Hope this helps.