Don't forget that there is only one D channel and all devices that want to transmit have to contend for it. This is where the NT1 helps. It is able to signal to one or other device (or to both) that a collision has occurred.

It is not predictable which device will be informed of the collision, or whether they both are. The device which is not informed of the collision will not be aware that this happened and continues to transmit on the D channel successfully. The other device stops transmitting. It then waits for the D channel to become free and tries again. By this time, it's too late because the network has awarded the call to the device which won the collision.