Exactly FLightDetent, that was the issue! During the RTO the FO will always call ATC (since the STOP command, CAPT will be PF no matter who was flying before) but that only will be a RTO call..
After full stop, ECAM actions, etc etc...AND if evacuation is required, another call to ATC will have to be made...and why can't this one be done exactly as in any other evacuation?
I guess since the FO did the first call for sure (unlike in any other evacuation where maybe there wasn't a previous call or it may have been CAPT or FO depending on who was PF), Airbus prefers the FO to continue handling ATC comms as FO knows exactly how much info was given in the first call and how much info is left to give...But again, it's only a guess, I really have no clue..
Thanks both for the help. FLightDetent I didn't know it was a relatively new thing on the FCTM...