I wholly disagree. The system as it works today is miles better than a circular buffer. People trying to leave new voicemails are informed that there is no more room and thus they know that if it is important they should try calling back later.

If it just deleted old entries then it's possible someone left an important message and assume you'll hear it but then it just gets silently deleted.

Imagine if PC storage devices worked this way, it'd be chaos.