As an entrepreneur, you need to accept that your telemarketer(s) / call centre(s) do not bring miracle.
What your telemarketing campaign will bring your back, is up to what your invest in it.
Namely, have you organized the whole marketing campaign?
If you want to have a preview about what a telemarketing will bring you back, well, take up your phone, keep on calling prospects, manage to create interests in your services / products, manage to sell.
Right after 4 hours of calls, you should be able to know whether a telemarketing campaign is useful for your business.
Telemarketing works best, as part of a whole, meaning, included in a global marketing campaign with regular mail campaigns, e-mail campaigns, social media / viral campaigns, door to door direct marketing.