Getting caught with a cell phone in prison is one of the worst disciplinary situations an inmate can land in, and the consequences stack on multiple levels.
At the facility level it is an immediate major infraction. That means disciplinary segregation, loss of privileges, and a serious entry in the inmate's record that follows them to every subsequent review. When that person sits in front of a parole board, the board sees everything, and a cell phone infraction signals exactly the kind of deliberate rule-breaking that boards are designed to screen for. It is not a minor paperwork violation. It demonstrates that the inmate made a calculated decision to circumvent facility security, and boards treat it that way. A hearing that might have gone well can go sideways fast with that on the record.
At the federal level the stakes are even higher. Under legislation signed during the Obama administration, possession of a cell phone by a federal inmate is a standalone federal crime carrying up to five years added onto the original sentence. Prosecution is not automatic, and in practice it is unlikely unless investigators can show the phone was used to conduct criminal activity on the outside, coordinating drug deals, witness tampering, running a fraud, anything like that. If that evidence exists, prosecution becomes very likely and the consequences are severe.
On your second question, yes, parole rehearings are a real mechanism. If someone is denied and given a future hearing date, whether that is two and a half years out or another interval, they can and should use that entire window to rebuild their record. Clean disciplinary history from that point forward, completed programming, and a stronger release plan all matter when that next date arrives. The board that denied them is not necessarily the board that sees them next time, and a clean stretch after a bad infraction does carry weight.