To add, if it is a digital station, you really need to make sure that the jack is only wired to the green/red pair on the jack (pins 3/4 on a 6P6C jack or 4/5 on an 8P8C). The second pair on digital sets was meant to interface outward to external devices, such as Wanderer handsets, etc. A long cable run, as is typical to classroom phones in a school can put all kinds of 'confusion' onto the second pair that will cause things like this. It could also be due to a physical fault on the second pair. Play it safe and just make sure that the white/blue pair is the only thing connected to a good jack and you'll be fine.

The phones and wall mount kits were shipped with two-conductor cords to protect against this (my assumption), but we all know that four-conductor line cords are the norm and this phone's cord may have been replaced along the line.

Ed Vaughn, MBSWWYPBX