In case of IP address restrictions, if you want to connect to SFTP via another host eg.a server, you can do this by SSHing in to the other host.
Open a Putty session and connect to your host using SSH.
Right click the tool bar and choose "Change Settings"
Connetion -> SSH -> Tunnels
Click "Add" - this will add "D443"
This opens a connection on port 443 on your machine to the Putty session,
Edit -> Settings
Open Connection -> FTP -> Generic Proxy
Type of generic proxy: SOCKS5
Proxy host: localhost
Proxy port: 443
Now you can connect to your SFTP host VIA the host you opened in Putty.