First thing I'd try would be swapping out two phones without changing any programming. If problem stays at the physical location it's a weird network issue (don't know what might be in the network) and IT can work on it. If problem moves with the phone extension it could be phone instrument or extension programming. Unlikely phone is bad but you could check by swapping MAC in programming for extensions, see if problem stays with extension or stays with instrument. More likely something is wrong with extension programming. You can search for the error. You can copy/paste from a working extension. Or I'd probably put a dummy MAC in existing bad extension, create a brand new extension and attach the MAC of problem phone and test. If it works I'd consider just fixing my test extension to be this guy's extension.
