This paper describes - from the study of a case - the problem of forecasting the service average time (SAT), for a given product, in the call center of a large Brazilian company in the sector - Contax - and how it was approached with the use of multiple regression with dummy variables. After highlighting and justifying the importance of the topic, the study presents a brief review of the literature on methods for forecasting demand and its application in call centers. The case is described, initially, contextualizing the company studied and describing, next, the way it deals with the problem of forecasting SAT for the product 103 - services related to fixed telephony. A multiple regression model with dummy variables is then developed to serve as the basis of the proposed forecasting process. This model uses available information capable of influencing SAT, such as the day of the week, the occurrence or not of a holiday, and the proximity of the due date of the telephone bill; and it presented an accuracy gain of 2 percentage points for the study period when compared to the tool previously used.