Check the coverage maps online, to ensure that you have a signal within the area you are currently in.
Phone line busy is a sign that your signal has deteriorated or you’re in a low coverage zone.
Remove and reinsert the iBurst Modem or switch off and on.
Install an antenna.
Reboot the PC.
Check if Ethernet port is OK (UTD) or that the dialup interface is enabled (UTC).
Call the support centre to assist further if the above has not helped.