One thing to keep in mind is that I'm sure that k7 didn't design the
service for use as a podcast feedback line. It's intent was as a
fax/voice mail service. Maybe the assumption is that if you're not using
it for a month, you don't care about it. I assume it costs them something
for each phone number they have registered. So they probably don't want
people having a number who barely use it.
Also, to echo what Steve Holmes said, you can always call the number yourself at regular intervals. That's what I do if there hasn't been any Darcy and Holly show feedback for a while.