He used to look for answers about the Matrix with his old friend Harold Hobbs, and after being freed by a redpill he goes to talk to him, leading to Harold being contacted by an Agent about his whereabouts and for him to be watched and followed. Jeff contacts him through his computer (like Trinity did to Neo in The Matrix), saying not to tell the Agent anything and that he'll be in contact. From the file the Agent has on him he appears to know Trinity.
He later calls a public telephone as Harold walks past telling him that he is no longer being watched (which actually appears not to be true) and to go home, where he once again contacts him through Harold's computer saying that the answers now look for him.
The fact that he seems to know Trinity combined with the way he contacts Harold a number of times and the way they used to look for answers together about the Matrix makes it look like he used to be a hacker.