An I-plate and an SSFP (Service Specific Front Plate) are different things.
The I-plate fits on the NTE5 and then the normal single socket front plate fits on the front of an I-Plate. You then plug your filter into the socket as normal.
The newest NTE5s (marked "Openreach" instead of "BT") already contain the I-Plate gubbins so adding an I-Plate to one of those is a waste of time.
An SSFP simply replaces the standard front plate to the NTE5 and contains a filter. There are two sockets, one for the telephone and one for the router. Hence it is sometimes referred to as a "splitter" (but not by BT!)