Custom ISDN – An ISDN protocol which pre-dates National ISDN-1. In most cases, National ISDN-1 is also available. The Northern Telcom DMS-100 switch supports “Custom DMS ISDN”. The AT&T/Lucent 5ESS switch supports “Custom Point-to-Point” (PTP) and Custom Point-to-MultiPoint (PMP). The ISDN protocol has no relation to where one may call. The Telos Zephyr and TWOx12 do not support PMP.