AT&T System 25 - Manual (Page 16)

Call Accountability
You can use your own extension number at any other extension
to place toll calls that should be billed to you.
Billing Calls to Your Extension
1 Listen for dial tone, then press
2 Dial your extension number.
3 Listen for dial tone, then dial the telephone number.
NOTE: If you make a mistake, hang up and restart the
sequence.
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