I run a telemarketing company and have worked long term on SAAS projects. Firstly, they must come accross well on the phone. I would get them calling in front of you at the interview stage. If they have the guts to do that and do it well while you watch then they will be good.
They need to be very tenacious, some days they wil make 100 calls and get no where. Or even several days in a row, but they have to be able to keep going. Not an easy ask, I've seen people reduced to tears after making 50 calls.
They also have to be clever enough to react in different way to different people and have the ability to interest people on the phone. I'd also say you would be best off paying a bit more for quality. In telemarketing the old saying if you pay peanuts you will get monkeys is true,
I hope this helps
Jonathan