Android supports L2TP/IPsec since version 4.2x. If you have an older version, you need to choose another protocol.
Open the system settings.
Be sure you are in "Connections" tab and tap "More networks" menu item.
Tap "VPN" in order to get to VPN connections settings.
On "VPN" screen tap "+" sign at top right side of screen.
Fill VPN Connection form with next values.
Type: L2TP/IPSec PSK
Server address: rs.trust.zone
IPsec pre-shared key: trustzone
Trust.Zone-Serbia rs.trust.zone trustzone
Tap "Show advanced settings" and be sure that "Forwarding routes" is set to 0.0.0.0/0. Then tap "Save".
If all goes well, you should see new VPN connection, so tap it if you want to connect.
Put your username and password and check "Save account information" on if you want Android to remember your login credentials. Then tap "Connect".
If connection has been successfully established, you will see "Connected" near your VPN connection name as well as "key" sign in notification area.
Your IP: x.x.x.x · Serbia · You are in TRUST.ZONE now! Your real location is hidden!