I don't use NET:telnet to see the active ISDN Connections.
because it takes al Lot of time and you don't have problems with the Authorisation
snmpwalk -v 1 -c Community YOURHost .1.3.6.1.4.1.9.9.27.1.1.3
if you have Cisco Routers you can use the Cisco OID Browser
http://tools.cisco.com/Support/SNMP/do/BrowseOID.do?local=en&translate=Translate&objectInput=.1.3.6.1.4.1.9.9.27.1.1.3