If the aircraft has been handed over from a unit which is an 'approved' one for verifying Mode c (there is a list), then no further verification is not required.
You mention 'free call' which suggests it is not handed over. In that case, how will we know that the Mode c is previously verified? Therefore verification is required.
We may also ask you to 'ident' so that the computer links your squawk to the flight plan if it has not done so automatically.
All of the above is from a London Control point of view.