Forum:Trap Priority

Ok so I was watching my friends play, and player A enters his battle phase, player B activates Threatening Roar, player A chains Dark Bribe, and then player B chains Trap Jammer

Now, player A says he cannot activate Trap Jammer because it isn't technically the battle phase, because he chained it to the change in phases. Then player B says he activated it when the battle phase started. Player A says he can attack first because he has turn priority then.

Somehow that sounds right, but I thought you could activate Threatening Roar at anytime, it doesn't have to respond to an event?