For Fax Overflow to operate:
set the fax machine up as an extension on the telephone switch, using an Analog Terminal Adapter (ATA).
acquire a central office (CO) line from the operating company, and configure the PBX so that this line appears at the ATA set (the fax extension).
Incoming fax calls ring at the fax extension. The fax extension is configured to Call Forward Busy (CFB) and Call Forward No-Answer (CFNA) to the CallPilot system extension, which answers to provide Fax Overflow functionality. Outgoing calls go to the default line or pool configured for the fax extension.
If the fax extension line is the same CO line, outgoing fax calls cause incoming fax calls to receive a busy signal from the CO. For this reason, configure a different line or pool for outgoing calls to keep the CO line free for incoming calls.
You can also configure several incoming lines to target the same fax extension, informally or as a DID trunk group. If there are multiple simultaneous calls to the fax extension, CFB directs the surplus calls to the Fax Overflow mailbox.