Brian Gaff brought next idea :
Well, I would advise you to have at least one old fashioned line powered
phone on the truecall for when you get a power cut. truecall defaults to
straight through, so this should work fine.

Good advice to have an ordinary phone, for emergencies and power cuts -
I have one, but with the ringer turned off.

Idea of the Truecall is to filter the calls, not let them straight
through unfiltered - letting just the calls through you want. If the
Truecall is built into the wireless phones, then calls would still get
through to the ordinary phone and cause it to ring, unless the ringer
is turned off.