It is rare but not rare
enough to get a person on the other end of a business call who's
pronunciation is impossible to understand. I always blame it on the
employer never on the employee.
Why is it the fault of the employer?
Because it is really inexpensive to send an employee to a class to learn how to speak national dialect English. TV English. In most areas of the country a one week course costs as little as $600 and the employee comes out speaking clearly in our national dialect, understandable to everyone.
$600 you miserable cheapskate company that won't train your employees.
Training employees to use our national dialect is cheap because it is easy. It involves listing the 10 main sentences used in phone conversations and the 500 common words used in specific business conversation. Repetition with audio playback is all that the employee needs to get the dialect right.