NEC 5140 - Manual (Page 31)

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Trunk Flash
The Trunk Flash feature allows you to access Centrex features (if available)
while you are talking on an outside call.
To flash a trunk while talking on an outside call:
1. Press Trans/Conf.
2. Dial *57 for a single flash or *56 for a double flash.
3. Wait for dial tone.
4. Dial the Centrex feature access code.