It's really simple arithmetic. If you pay your sellers to close deals and they make, say $200k, why would you want them to spend time doing what you can hire someone for $60k to do?
I've used the outside "appointment setter" companies. They usually charge roughly $700 per appointment. So, do the arithmetic again. If you close one in, say four, then add $2800 into your cost of sales and see if it makes sense.
Looking at the current state of market, I have seen few very good companies assisting in appointment setting but unfortunately a number of new companies who have come into the market and have somewhat created a bad name of these so called "god ones". So your biggest challenge would be get hold of one and if you manage, its a win-win situation. If not, as rightly pointed out by others, it can go waste.
So decide wisely and do in-depth research to find your right partner.
The other way round is to hire a good resource and get hold of quality database, which I am sure can help.