Ironic as BT and AT&T did like each other once...
Once upon a time, well probably about 1999, BT and AT&T (that's the old proper one, not the new at&t) did actually form an alliance. The plan was that they wouldn't step on each other's toes in the other's home market, and they would share stuff:
The fount of all knowledge has some info: https://en.wikipedia.org/wiki/Concert_Communications_Services
At the time I worked for AT&T Communications (UK) Ltd, which was a proper UK-based telco. Of course no-one had even warned us of this tie-up, so in one foul swoop of a press conference/release we all realised our days were numbered. I left soon after and what was left was flogged off to Viatel and/or Global Crossing I recall.