Think of the long term consequences with regard to your present relationship that is doing good.
Think also that talking again to your ex may give a temporary immediate pleasure but will leave you only more and more frustrated since with the changed conditions you cannot go all the way.
Think of the difficult times that led both of you drift from each other.
Think of the complications and the emotional drain as well as confusion it is likely to create.
Remind yourself of all these and resist the temptation. It may however be good to see what caused this urge to call. It is always helpful to grow conscious.
Affectionately,
Alok Da