3Com 3C410012A - OfficeConnect Remote 531 Access Router Getting Started Manual page 49

Interface Cable Characteristics A-5
V.24/V.28 WAN Cable
The WAN port can be configured to support V.24 or V.28 signaling
at data transfer rates up to 19.2 Kbps. Note the following pinouts
when buying or making up a suitable WAN cable.
Autosense pins must be connected in order for the cable to
function correctly.