Regarding the references it is only one of each, it will be stated in the package they send you around a week before the assessment.
And regarding the phone call it seems that besides confirming all the info submitted to them previously (CPL, IRME, MCC, medical dates, flying hours...) it is also a quick english level assessment conversation as some people seem to have more questions asked than others.
Anyway nothing to stress about, just have your info ready at hand as the call can come anytime
Cheers,