The idea of sustainable development links business, environmental and social objectives into one integrated effort aiming to meet a common goal. Due to the influence of current developments in customer behaviour companies revise their business strategies towards more sustainably oriented ways of production, business practices, resource efficiency, waste disposal, building partnerships, communication effectiveness etc. Market with telecommunication services is affected by technology development. Growing demands of customers result in re-evaluation of marketing routine. The main purpose of the research is to identify the change in customers’ approaches and satisfaction with offered products, enterprise processes, and business strategy as a result of implementing Customer Relationship Management in 2014 and subsequently in 2018 with the dependence on socio-demographic characteristics. Customers’ opinions were investigated using a questionnaire. Respondents were selected by direct sampling with contingent valuation. Sample set consisted of 3,282 respondents. Following the results, the fact that there are changes in approaches of customers in investigated areas in terms of gender and age as a result of implementing the Customer Relationship Management can be stated. The customers’ satisfaction with provided services improved. In order to serve the new and constantly evolving needs of the growing number of customers who care about social and environmental issues, the companies are required to permanently monitor their performance on the market and incorporate customer feedback as in input for evaluation of their internal processes. In terms of marketing, positive changes in approach, satisfaction with the quality of provided services and reduced service time were observed. In terms of business strategy, the changes in connection strategy, the effect of customer-driven strategy and services provided by call centres abroad are perceived in positive way. Keywords: sustainability, economical sustainability, marketing, Customer Relationship Management, telecommunication company.