Although you are right about the improper ringing, REN is a figure of merit for the indicated capacitive loading of a device (such as a modem or phone) connected to a twisted pair local loop, behind the primary socket box supplied by BT.

If a device can be connected to a phone line in the UK, then it should have a BABT approval sticker, either in the book on on the device with the REN value stated.

There is a bit of a dodgy grey area when you talk about TPL devices without a ringer feed line, though...

One of the few remaining Mk1 owners... #00015