“The plan you're on”
Be on a plan means to subscribe to a particular service package. The plan you're on is the customer's current plan, a common expression for mobile-phone or insurance contracts.
A2-53 / CEFR A2 / EPISODE 53
Checking a mobile phone plan
SceneAt a phone shop. Frank discusses plans while Tina waits beside him.

WORDS & CONTEXT
Be on a plan means to subscribe to a particular service package. The plan you're on is the customer's current plan, a common expression for mobile-phone or insurance contracts.
An amount followed by off marks a discount. Fifteen dollars off the old price means fifteen dollars less than the previous charge.
Ma'am politely addresses a woman; see A2-28. The Japanese translation renders it as a courteous address to the customer.
Don't worry. means there is no need to be anxious, and won't change means will stay the same. After the staff member's explanation, Frank reassures Tina, who was afraid they would lose touch.
Does that make sense? checks whether an explanation was understood, and Tina repeats back the points about the price and the number. However, forever is Tina's relieved exaggeration. The staff member only said the number will not change with this price change, not that it will be the same number for all time.
Get a day off means to be given a day of rest. Tina has memorized Frank's number, so if it does not change, she does not have to learn a new one. She describes that relief as her brain getting a day off. The punchline is the difference in what matters to each: Frank is glad about the discount, while Tina is even happier not to have to memorize anything new.