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