Incoming faxes on a Trunkline with detection enabled should be detected by asterisk. You may need to increase the ''Fax Detect Delay'' in ''Global Settings'' of SAIL to ensure the fax is detected and routed to the fax extension before the call is routed anywhere else.
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Once detected the incoming fax should be routed to the ''Autosense FAX Extension'', where IAXModem will answer and pass the fax to Hylafax.
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Outgoing faxes emailed to 'user@0123987654.fax' will be sent by Hylafax to IAXModem with a dial string prefixed by 99, for example 990123987654. This will initiate a call in asterisk. The 99 prefix will be recognised as the fax route and the call passed to the relevant outgoing Trunk, where the 99 is stripped off and the real number dialled.