Final answer:
Mike is experiencing retroactive interference, where his new cell phone number interferes with the ability to recall the old number.
Step-by-step explanation:
When Mike gets a new cell phone and can only remember his new cell number, not the old one, this is an example of retroactive interference. Retroactive interference occurs when more recently learned information hinders the recall of older information. In contrast, proactive interference is when old information hinders the recall of newly learned information.