Check if the discount is off the total bill amount, or just one part of the bill. Also, make sure you check the prices as there may be another plan with no discounts but a lower overall tariff rate.
You might have a plan that has a certain 'benefit' attached to it for a period of time (this is usually called 'benefit period'). Usually this is a price discount, but it can also be items like movie tickets or loyalty points. After the period ends, you will either pay full price again or no longer enjoy your benefits.
For example, you might sign up to a plan with a 12-month benefit period with a 10% discount. After 12 months, you stay on the same plan but no longer receive the discount. Retailers must tell you that your benefit period is ending at least 20 business days prior, and no earlier than 40 days.