3Com 3C410012A - OfficeConnect Remote 531 Access Router Getting Started Manual page 25

Introduction 1-9
Incoming ISDN traffic carries both a voice and data identifier and
depending on the type of transmission, voice calls are automatically
directed to the appropriate port.
The Voice port has an RJ11 socket. If your telephone equipment uses
a different connector, you will need to obtain a suitable convertor to
connect to the Voice port. A British Telecom to RJ11 convertor is
supplied with UK models.