This paper examines the impacts of mergers and acquisitions (M&As) on mobile network operators (MNOs) based on a cross-country firm-level dataset over the period 2007Q1 to 2020Q4, in which OECD countries have been more reluctant to approve mergers, and thus provide a quasi-experimental setting for a difference-in-differences (DiD) test. From the M&A cases of 94 mobile operators in 29 OECD countries, our results indicate that the post-merger behavior of MNOs in pricing and investments are heterogeneous depending on the market conditions, whereas the results of the entire sample show mixed results. Specifically, we find that prices at merged MNOs increase only in markets with low competitiveness and low subscriber density. Also, post-merger investments by MNOs decrease with high-competition and low subscriber density. This paper provides informative results for future antitrust decision making in the mobile telecommunications industry.