Check the telephone 'mode' switch.
Usually located on the underside on most modern instruments, there is a small switch to determine how the phone signals to the system. This may have the settings LD/MF, or possibly MFE, MFT or TB.
Depending on how the system has been configured and what extension card is being used an extension could operate MF with 'earth recall', MF with 'TB' or even LD. Depending on the settings presented on your particular phone normally the MF or MFE position should suffice. MFT & TB relate to 'timed break' recall.