Companies in the manufacturing and service industries often face significant workforce management challenges due to demand variability and unpredictable absenteeism. These factors can result in overstaffing or understaffing, which increases labor costs and reduces service quality. In the context of evolving labor regulations, such as reduced working hours, flexible staffing strategies have become increasingly important. This paper evaluates a workforce flexibility approach that integrates three elements to improve adaptability: annualized hours, the hiring and firing of temporary workers, and multiskilling. To address demand uncertainty, we designed a two-stage stochastic optimization approach and validated it using operational data from a Colombian outsourcing company. We use Monte Carlo simulation to test the performance of the proposed approach under different demand fluctuation scenarios. The results show that annualized hours and multiskilling satisfy a significant share of total demand. This reduces the need for temporary contracts during peak weeks. The combined approach improves labor cost efficiency, reduces deviations from optimal staffing levels, and helps maintain service quality by providing robust staffing plans, even in situations of high variability. These findings highlight the importance of incorporating flexible labor practices into workforce planning, particularly in the face of regulatory changes and unpredictable demand.