Biased opinion here but I'll give it a shot. I run a virtual assistant company that does cold calling for Real Estate, PowerISA. Here is what I say will have the most impact on your ROI from using a VA to cold call for you:
1) You need to plug in the VA into a system that works. Are you already closing deals using the same lead source and tools? If not, you're gonna run into many more issues that will take longer to fix.
2) What is your lead gen method? if you're using mailers to generate responses, your VA will have a much higher conversion rate than straight cold calling. Generating Google PPC leads for all cash offers? Conversion rate will be the highest.
3) Your VA needs to be good. They need to have experience cold calling. It's super hard to teach a noobie how to cold call or better said, you need to go through a lot of them to find one that can handle it. Talent matters. A great VA will last 6-12 months in a cold calling role.
Let me know if this helps.