Hello,
Have you checked the cable between the phone and switch (are the wires in correct sequence on both ends?). Is there more IP phone or PoE device connected to this switch,if yes do you expereienced the same with them? Can the switch overloaded in PoE aspect ( if there are several PoE device).
I guess this is a cable problem or RJ-45 jacket...
bye
FCS
Please rate me if I helped.