Flat rates are a prominent pricing scheme for telecommunications services and are often preferred by consumers although average costs would be lower in an alternative usage-based tariff. Reasons are that flat rates protect against unexpectedly high costs (insurance effect), are more likely to be chosen if actual usage is overestimated (overestimation effect), and prevent any disutility that is associated with the immediate perception of marginal costs (taximeter effect). This study complements the literature on tariff biases by highlighting that a lack of tariff flexibility is a major impediment to choosing a flat rate: empirical support for this flexibility effect is found, while, at the same time, the insurance and overestimation effect that run in favor of flat rates are confirmed. Finally, the managerial implications of the findings for the introduction of the new cost cap tariff are discussed. The hybrid cost cap tariff can combine the flexibility and the insurance property, and may, therefore, exert a cost cap bias on consumers.