Cat5 is of course RJ45 rather than RJ11, but I have seen (in the distant past) RJ45 sockets used to provide an RJ11 plug with comms.
TBH, the op's post looks more like a serial to RJ45 converter, I still see those in places - doesn't convert to ethernet - simply changes the connectivity in order that serial can be more easily cabled up elsewhere using premade ethernet cables.