I have three family sims, I notice that 2 of these are showing charges for UK texts, are these not included? The charges are not large, one is 40p and one 25p
Fire up MyBT, click on "Your usage" and you can see exactly which calls each of the SIMs have made and when. You will be able to check whether they are included in the "allowed" (free) texts.
Apologies, my post crossed that of @StewM