Your reference can't speak
Some organizations have strict policies that prohibit employees from speaking on behalf of former colleagues. In these cases, a reference may only be allowed to divulge the persons dates of employment. Thats likely not going to be enough information for your prospective employer.
In this instance, some people choose to use personal references instead, but prospective employers dont want to talk to your friends or relatives. They want to talk to people who managed you or who have worked with you.