Final answer:
The student's problem with remembering old phone numbers is an example of retroactive interference, which is when new information hinders the recall of older memories.
Step-by-step explanation:
The student's difficulty in recalling their previous phone numbers while easily remembering their current one is most closely related to the concept of retroactive interference. This occurs when new information, like a current phone number, interferes with the ability to recall older information, such as previous phone numbers. This kind of interference is common and illustrates how newly acquired knowledge can inhibit the retrieval of older memories.