Sounds like your network isn't configured correctly, this is quite a common error, first steps just to ensure it's not a faut with your installation is to reinstall your modem and drivers.
Once this is completed if you are still experiencing the problem contact your ISP, if you are on an IPStream backbone the fault is likely to be with BT, if you are on a Datastream backbone you will be travelling across your ISP own network, in which cases they will need to check your scripting and possibly escalate the case to their network support tam dependant on their protocols and network access priveledges.
Hope that helps!