These days there is an emphasis on keeping writing as short and simple as possible. It's vital to proof-read any letters or documents that you are sending out by post, because mistakes convey a poor impression of both you and the company. When telephoning, it's a good idea to have brief notes in front of you when you are making an important phone call, so that you don't forget the points you want to get across. If possible, try to form a relationship with the person on the other end of the line. Unfortunately you may sometimes have to deal with complaints or criticisms from people both inside and outside your organisation.
Communicating is as much about listening as it is about talking.