Best thing to do is request copy of all notes made at these meetings. Employment law according to the Citizens Advice says " If an employer has excepted poor standars in work previously then it is classed as unfair dismissal" If no previous warning has been given then again it is unfair. They seem eager to stay out of a tribunal & if you win the tribunal can request the charge to be removed. Good Luck. Power to the People

It is possible to argue the fact that the issue arose due to the amount of stress that you was under at the time of the call. Also how the customer difficult nature caused your stress to escalate. A moment of uncontrolable madness. You could say you was not refering to the customer but something else that happened like computer freeze.