Assurances of Effort
Customers don’t like feeling as if you are not making an effort. On the other hand, when customers feel your efforts go above and beyond that which is expected from them, they are less likely to resort to angry behavior.
You can express assurances of effort in a variety of ways, such as “. . .” I promise not to be this long next time.”
The assurance that you will try is different from being able to assure the customer of a particular result. The former can be offered even if the latter is not yet possible.