Phil, it also depends upon how old the model of digital sets your talking about are.
1st Gen Aries phones (M2616, M2008, etc) usually will let you know if they're plugged in or not (you won't get the info you get with IDU) by using, as BillCorv said, UNTT in LD 30 but they will not respond to an IDU.
The earlier digital phones (M2008, M2112, etc) and, if your system is old enough, the analog/digital hybrid SL-1 phones also will not respond to an IDU.
Regards,
Harry
"The legitimate powers of government extend to such acts only as are injurious to others. But it does me no injury for my neighbour to say there are twenty gods, or no god. It neither picks my pocket nor breaks my leg." --Thomas Jefferson