If we bumped it to 50,000 pieces a month, all calling would have to go through a call center or VAs.

Much of the Tenant-Buyer pre-screening can be farmed out to VAs who you pay to answer the phone (Google Voice, Vumber, etc.) that you set up to receive calls from your marketing. 

Might seem like a lot of work, but it pays off and when you get to a certain point you can just hire VAs to do that part for you.

Use VAs to post ads on Craigslist.  

I'm looking for a company that I can rely on staffing so I don't have to keep on hiring and retraining VAs.

Of course, if your profits allow, you could spend a ton on a few “qualified” VAs (good luck with that) to do all that for you.

We have been using several different VAs to do our cold calling for us for about a month now, and they are not producing, their dials per hour are always off and we cant seem to find out why. 

Of course, these VAs aren't licensed agents so the extent to their conversations is pretty much a survey to identify their motivation, where I (or ISA) would reach out to start the conversation.

@Andrew LeBaron thank you so much...VAs appear to be the right answer!

BUT upwork.com is a good site for VAs.