Final answer:
The phrase 'not only' is a correlative conjunction used to connect two balanced clauses, emphasizing that Jason exceeded his data plan limit and also missed his cell phone bill payment.
Step-by-step explanation:
In the sentence "Not only did Jason go over the limit of his data plan, but he also forgot to pay his cell phone bill on time," the phrase "not only" is used as a conjunction that introduces the first of two negative statements that will be connected by "but also." This is part of a correlative conjunction pair, which is used to link together two balanced clauses, phrases, or words. In this case, it emphasizes that Jason committed two mistakes: exceeding the limit of his data plan and failing to pay his cell phone bill on time. The second mistake is introduced by the phrase "but he also," which works in coordination with "not only" to give both parts of the sentence equal importance.