As per RFC 4006, Validity-Time is used to indicate to the credit-control client how long the subscriber is allowed to use network resources before a new intermediate interrogation is sent to the server.
the server is an online charging system , the client is the PCEF.
Are there any benefits from adding a randomized value to CCA Validity-Time in order to reduce reporting storm to PCEF for voice calls? I think there is for non - duration grants - i.e data since number of data sessions are larger than voice calls.
I don't think it would be necessary, unless you are expecting a large number of calls to to be originated at exactly the same time, and even then the original call setup would probably be a bigger load which your system would have to handle anyway.
If the validity time is constant and the call originations follow some normal distribution, then the intermediate interrogations should follow the same distribution.