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By default, the intercom feature on this phone is set to 'Tone Call', instead of 'Voice Call'. You may change this if you wish - it means that a tone (like a ring-tone, but different from the usual) signals that an intercom call is being signaled for your phone station.
To call another extension in your phone system, you first pick up the handset (take the phone off-hook) then dial the extension number.
A tone will sound at that extension, to signal that an intercom call is incoming and the person at the station should pick up on the call.
The puzzle might remain, as to which extension numbers go with which phones.
The extension numbers, depending upon your office's phone network and phone system, may be assigned either by the port number on the phone system box, perhaps, or more likely by the programming and assignment in the phone system box itself (whatever make and model it is - and to advise you further I would have to know that, at minimum.)
One way to find out what extension is which extension number: practice trying to call another extension while the office is closed... maybe with another person to listen for the 'intercom ring tone' and locate which phone it came from, and tell you. So that way you could match up extension numbers with phones.
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