Are there systems or methods known which might be used to help me know
if a telephone caller is someone who has called before?  The caller
will not have taken part in any formal learning session for, say, a
prompted text-free speaker recognition system.  Instead, the caller
may have spoken for a minute or two, perhaps answering questions,
perhaps repeating a few prompts, but that's it.  Identifying that
someone might have called before does not need to be highly accurate,
just an estimate. I would expect that such an estimate might be more
accurate if we had a larger voice session the first time they called.