ITT 400E card is the best design of 400-type cards (which was as well admitted by Bell System/Western Electric based on several quality/trouble statistics done by operational compaties). I have 30 of ITT 400E cards in my possession and I have never come across a faulty one. SanBar 4000F I have as well and there are some faulty.

The design of ITT 400E card is based on single side PCB (which is much easier to repair vs. double sided). It only has basic discrete components - not like WE 400H with custom logic IC. Out of active components it has a single generic operational amplifier IC (8 pins) and 2 or 3 transistors. This should be very repairable.

I use WE 400H which has a single advantage of being silent (click-free) as there is only a single reed relay used in that, and ITT 400E which has an extra benefit of automatic release of abandoned incoming calls from hold behind my PBX. Abandoned outgoing calls still have to be released from hold manually.

If faulty - I would first check and replace the electrolyte capacitors, those are the most aging components especially when not charged for a long time. If this doesn't help I would replace the transistors. And if the issues is not with those - change the Op.Amp. IC. I am assuming that the relays are checked and contacts are clean.