This read-create DisplayString holds the Service Profile Identifier (SPID), a 9-20 digit numeric string, that some ISDN switch protocol types such as Bellcore NI-1 and NI-2 require in addition to the local number, and is a zero-length string if not needed. An admin configures it when the ISDN provider's switch type requires SPID-based terminal registration for the line to come up. Leaving isdnEndpointSpid empty on a switch type that requires it would prevent the ISDN terminal from ever successfully registering on the line.