PBX => PBX Configuration => Extensions => Generic IAX2 Device
then
Fax => New Virtual Fax
then
PBX => PBX Configuration => Inbound Routes
and assigned your Fax Extension to your Zaptel Channel.
you can create Fax Lines as you like
Example:
IAXMODEM1 Zaptel Channel 1 Fax Line 1
IAXMODEM2 Zaptel Channel 2 Fax Line 2
IAXMODEM3 Zaptel Channel 3 Fax Line 3