Something
public text v1 · immutable[fax-in]
exten => fax,1,Answer
exten => fax,2,Set(TIMEOUT(absolute)=300)
exten => fax,2,Set(FAXFILE=/var/spool/asterisk/fax/i${CALLERIDNUM}-${TIMESTAMP}.tif)
exten => fax,3,rxfax(${FAXFILE})
exten => h,1,hangup
exten => t,1,hangup
[default]
exten => fax,1,Goto(fax-in,s,1)