Atleast name ur ISP man,
every ISP has a different set of architecture. airtel on its ADSL can always be used for FTP, nothing is blocked from elsewhere other than the CPE MODEM they install at ur end. U can use another Dlink/Netgear router in which u can configure the serrvice port forwarding easily. But u will need to track ur IP regularly as it changes everytime u re-connect.