A well constructed letter should grab the attention of the person you send it to and will achieve better results than a phone call, because calls can be intercepted by the automated service or the receptionist. Sending a letter to the proper person is crucial, think big, so big that you would never reach them by phone and they probably don't even sort their own email box.
When speaking person to person you may be prevented from getting your point across, but in a letter no one is interrupting you, your points are there for reviewing and the recipient has time to ponder the next step.
