Canon imageCLASS MF6540 Basic Operation Manual page 136

6
Talk through the handset to the recipient.
If you press [Hook] in step 4, you can pick up the handset when you hear the recipient's voice.
If you hear a high-pitched signal instead of the recipient's voice, go to step 8.
7
Ask the recipient to set their fax machine to receive faxes.
8
When you hear a high-pitched signal, press [Start], then hang up the handset.
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