Sounds like the extension needs to be programed to ring for incomming calls.
Programming is done on M7310, M7324 and T7316 sets.
To set up ringing you need to enter
programming by pressing the feature key followed by **266344.
When prompted to enter the password enter 266344.
The screen should say TERMINALS&SETS. The 4 top line/feature keys are now used for programming.
At the terminals&sets prompt, press the show key (top right arrow).
The display should now ask for an extension number.
Enter extension number you wish to program ringing on.
Press the show key (top right arrow)
The display should sat LINE ACCESS
Press the show key (top right arrow) .
The display should say LINE ASSIGNMENT.
Press the show key (top right arrow) .
The display ask for a line number. press list
The display should say line 001: appear only
press the Chng key till it says appear & ring
press next key (lower right arrow)continue with the remainder of the incoming lines that you have on your telephone
press the release key when finished