One solution could be to have different types of callers (similar to freelance marketplaces) to chose from. You could pick a less experienced/expensive caller for stuff that isn't really important and pick a more qualified/expensive caller for stuff that really matters. Callers could even specialise in certain industries.
Another solution could be to let users listen in on calls and provide live feedback to the caller but then most people would just opt to do the calls by themselves. Personally, I might still use such a service simply because I hate talking on the phone and am not good at sales.
If the calls being outsourced are sales calls, you could pay a commission. That would lead to more motivated sellers at least, although perhaps too motivated and too short sighted.