Error message: subscriber not exist

Incoming calls from certain numbers receive error message when trying to call my number, saying that subscriber does not exist. The problem is erratic but most commonly occurs through my work switchboard. I have logged multiple calls with Glocell & they don't reply/ the responsible manager does not get back to me as promised. I am a doctor at a hospital responsible for resuscitating sick babies. I have arrived late on the scene due to this problem.