Pleased to report all three sims can now call each other. The issue has been rectified.
Took 15 days to sort out.
No feedback to indicate what has been done or to inform us that the issue has been rectified.
This is the most annoying thing with Plusnet customer service; they just do not contact you at all to let you know anything. If they implemented a system where feedback was given as to what is being done or has been done, expected resolution etc. then this would greatly diffuse customer frustration.
Now need to request compensation....