In my experience, customers tend to become abusive or threatening if and when they feel that no one is listening to them, or simply if the agent's intellect is not up to par. This really frustrates callers and to manage this, agents simply need to listen. I'm sure there's a call flow process in every call center, as Managers, we need to ensure that our agents follow the call structure i.e welcome => obtain issue => offer solution => offer alternative => offer further assistance => cross-sell/upsell at the right opportunity => farewell. If this is followed correctly, customers will not see the need to call back and there is no frustration. Also, agents should not attempt to talk customers through problems they cannot solve, if you don't know what to do, pass it to someone who does - customers will appreciate this. However, i agree that there are some customers that do not accept 'NO' nor appreciate straight-forward service and will start being abusive or threatening when they don't get their way. I agree that their calls should be terminated, however, at least with a warning issued earlier in the call.